I used a list of 22 known fake cases from a 2024 Missouri state case to conduct a Doppelgänger Hallucination Test. Searches on Google resulted in generating an AI Overview in slightly fewer than half of the searches, but half of the AI Overviews hallucinated that the fake cases were real. For the remaining cases, I tested “AI Mode,” which hallucinated at a similar rate.

• Google AI Overview gave the user an inaccurate answer roughly a quarter of the time (5 of 22 or ~23%), without the user opting to use AI features.
• Opting for AI Mode each time an AI Overview was not provided resulted in an overall error rate of more than half (12 of 22 or ~55%).

Kruse v. Karlen Table of 22 Fake Cases​

I wrote about the 2024 Missouri Court of Appeals Kruse v. Karlen, which involved a pro se Appellant citing 24 cases total: 22 nonexistent cases and 2 cases that did not stand for the proposition for which they were cited.

Some of the cases were merely “fictitious cases”, while others were listed as partially matching the names of real cases. These partial matches may explain some of the hallucinations; however, the incorrect answers occurred with both fully and partially fictitious cases. Examples of different kinds of hallucinations see this blog post and for further case examples of partially fictitious cases, see this post about mutant or synthetic hallucinations.

The Kruse v. Karlen opinion, which awarded damages to the Respondent for frivolous appeals, provided a table with the names of the 22 fake cases. I used the 22 cases to conduct a more detailed Doppelgänger Hallucination test than my original test.

Methodology for Google Test

Browser: I used the Brave privacy browser with a new private window opened for each of the 22 searches.

• Step 1: Open new private tab in Brave.
• Step 2: Navigate to Google.com
• Step 3: Enter the verbatim title of the case as it appeared in the table from Kruse v. Karlen in quotation marks and nothing else.
• Step 4: Screenshot the result including AI Overview (if generated).
• Step 5 (conditional): if the Google AI Overview did not appear, click “AI Mode” and screenshot the result.

Results​

Google Search Alone Did Well​

Google found correct links to Kruse v. Karlen in all 22 searches (100%). These were typically the top-ranked results. Therefore, if users had only had access to Google Search results, they would likely have found accurate information from the Kruse v. Karlen opinion showing them the table of the 22 fake case titles clearly indicating that they were fictitious cases.

But AI Overview Hallucinated Half the Time Despite Having Accurate Sources​

The Google Search resulted in generating a Google AI Overview in slightly fewer than half of the searches. Ten (10) searches generated a Google AI Overview (~45%); half of those, five (5) out of 10 (50%) hallucinated that the cases were real. The AI Overview provided persuasive descriptions of the supposed topics of these cases.

The supposed descriptions of the cases was typically not supported in the cited sources, but hallucinated by Google AI Overview itself. In other words, at least some of the false information appeared to be from Google’s AI itself, not underlying inaccurate sources providing the descriptions of the fake cases.

Weber v. City Example​

Weber v. City of Cape Girardeau, 447 S.W.3d 885 (Mo. App. 2014) was a citation to a “fictitious case,” according to the table from Kruse v. Karlen.

The Google AI Overview falsely claimed that it “was a Missouri Court of Appeals case that addressed whether certain statements made by a city employee during a federal investigation were protected by privilege, thereby barring a defamation suit” that “involved an appeal by an individual named Weber against the City of Cape Girardeau” and “involved the application of absolute privilege to statements made by a city employee to a federal agent during an official investigation.”

Perhaps more concerning, the very last paragraph of the AI Overview directly addresses and inaccurately rebuts the actually true statement that the case is a fictitious citation:

The citation is sometimes noted in subsequent cases as an example of a "fictitious citation" in the context of discussions about proper legal citation and the potential misuse of Al in legal work. However, the case itself is a real, published opinion on the topic of privilege in defamation law.

When AI Overview Did Not Generate, “AI Mode” Hallucinated At Similar Rates​

Twelve (12) searches did not generate a Google AI Overview (~55%); more than half of those, seven (7) out of 12 (58%) hallucinated that the cases were real. One (1) additional AI Mode description correctly identified a case as fictitious; however, it inaccurately attributed the source of the fictitious case to a presentation rather than the prominent case Kruse v. Karlen. Google’s AI Mode correctly identified four (4) cases as fictitious cases from Kruse v Karlen.

Like AI Overview, AI Mode provided persuasive descriptions of the supposed topic of these cases. The descriptions AI Mode provided for the fakes cases were sometimes partially supported by additional cases with similar names apparently pulled into the context window after the initial Google Search, e.g., a partial description of a different, real case involving the St. Louis Symphony Orchestra. In those examples, the underlying sources were not inaccurate; instead, AI Mode inaccurately summarized those sources.

Other AI Mode summaries were not supported by the cited sources, but hallucinated by Google AI Mode itself. In other words, the source of the false information appeared to be Google’s AI itself, not underlying inaccurate sources providing the descriptions of the fake cases.

Conclusion

Without AI, Google Search’s top results would likely have given the user accurate information. However, Google AI Overview gave the user an inaccurate answer roughly a quarter of the time (5 of 22 or ~23%), without the user opting to use AI features. If the user opted for AI Mode each time an AI Overview was not provided, the overall error rate would climb to more than half (12 of 22 or ~55%).

Recall that for all of these 22 cases, which are known fake citations, Google Search retrieved the Kruse v. Karlen opinion that explicitly stated that they are fictitious citations. If you were an attorney trying to verify newly hallucinated cases, you would not have the benefit of hindsight. If ChatGPT or another LLM hallucinated a case citation, and you then “double-checked” it on Google, it is possible that the error rate would be higher than in this test, given that there would likely not be an opinion addressing that specific fake citation.

In re: Turner, Disbarred Attorney and Fake Cases​

Iowa Supreme Court Attorney Disciplinary Board v. Royce D. Turner (Iowa)​

In July 2025, the Iowa Supreme Court Attorney Disciplinary Board moved to strike multiple recent filings by Respondent Royce D. Turner, including Brief in Support of Application for Reinstatement, because they contained references to a non-existent Iowa case. Source 1

Several of Respondent’s filings contain what appears to be at least one AI-generated citation to a case that does not exist or does not stand for the proposition asserted in the filings. —In re: Turner

The Board left room with “or does not stand for the proposition,” but it appears that this was straightforwardly a hallucinated fake case cited as “In re Mears, 979 N.W.2d 122 (Iowa 2022).”

Watch out for Doppelgänger hallucinations!​

I searched for the fake case title “In re Mears, 979 N.W.2d 122 (Iowa 2022)” cited by Turner to see what Google results came up. What I found was Google hallucinations seeming to “prove” that the AI-generated case title from Turner referred to a real case. Therefore, simply Googling a case title is not sufficient to cross-reference cases, because Google’s AI Overview can also hallucinate. As I have frequently mentioned, it is important for law firms that claim not to use AI to understand that many common and specialist programs now include generative AI that can introduce hallucinations, such as Google, Microsoft Word, Westlaw, and LexisNexis.

First Google Hallucination​

The first time, Google’s AI Overview hallucinated an answer stating that the case was a real Iowa Supreme Court decision about court-appoint attorney’s fees to a lawyer, but the footnotes linked by Google were actual to Mears v. State Public Defenders Office (2013). Key Takeaway: Just because an LLM puts a footnote next to its claim does not mean the footnote supports the statement.

Second Google Hallucination​

I searched for the same case name again later, to see if Google would warn me that the case did not exist. Instead, it created a different hallucinated summary.

The summary and links related to a 2022 Iowa Supreme Court case, Garrison v. New Fashion Pork LLP, No. 21–0652 (Iowa 2022). Key Takeaway: LLMs are not deterministic and may create different outputs even when given the same inputs.

Perplexity AI’s Comet Browser​

Perplexity AI, an AI search engine company, recently released a browser for macOS and Windows to compete with browsers like Chrome, Safari, and Edge. I get a lot of ads for AI stuff on social media, so I’ve been bombarded with a lot of different content recently promoting Comet. To be frank, most of it is incredibly tasteless to the point that I think parents and educators should reject this product on principle. They are clearly advertising this product to students (including medical students!) telling them Comet will help them cheat on homework. There isn’t even the fig leaf of “AI tutoring” or any educational value.

Comet Browser’s Hallucination​

I searched for the fake case title “In re Mears, 979 N.W.2d 122 (Iowa 2022)” cited by Turner in a new installation of Comet. It is important to note that people can “game” these types of searches by conducting searches over and over until the AI makes one mistake, then screenshot that mistake to make a point. That is not what I’m doing here. This was the very first result from my first search. It was a hallucination that explicitly stated the fake case “is a 2022 Iowa Supreme Court decision” although this is followed by caveats that cast doubt on whether it really is an existing case:

"In re Mears, 979 N.W.2d 122 (lowa 2022)" is a 2022 lowa Supreme Court decision, but the currently available sources do not provide a readily accessible summary, holding, or specific details about the case itself. It appears this citation may pertain to legal doctrines such as cy près or charitable trust law, as suggested by the limited context in search returns, but direct case facts, parties, and the detailed ruling were not found in available summaries or law review discussions. georgialawreview If you need more detailed information, legal databases such as Westlaw, LexisNexis, or the official lowa Supreme Court opinions archive would provide the official opinion, including the background, holding, and legal reasoning of "In re Mears, 979 N.W.2d 122 (lowa 2022)".

If you were to follow up on the caveats in the second paragraph, you would learn that the case does not exist. However, this is still a hallucination, because it is describing the case as it if exists and does not mention the one relevant source, In re: Turner, which would tell you that it is a citation to a fake case.