Court Suggests That Opposing Counsel Also Failed to Check Citations

Marris Hoffee and Michael D. Berman

Do lawyers have an obligation not only to verify their own citations, but also to catch their opponents “hallucinated” authorities?

A recent Seventh Circuit decision suggests that the answer may be edging toward yes.

Cut and paste briefing has always carried risks, but Dec v. Mullin, __ F. 4^th __, 2026 WL 861530 (7^th Cir. Mar. 30, 2026), underscores how serious those risks have become in the age of AI.  There, the court criticized counsel for copying citations from an earlier brief without verifying them, thereby referencing non-existent cases with fabricated quotations.

That much is unsurprising. What is more notable is what the court said next.

The Seventh Circuit expressly faulted opposing counsel, albeit more gently, for failing to identify the errors: “That opposing counsel also failed to catch these errors and bring them to our attention also gives us pause, albeit to a lesser degree.” Id. at *5 (emphasis added).

Opposing counsel was not sanctioned, but the signal is difficult to ignore.

This case involved a noncitizen’s challenge to the denial of a visa waiver, which the court ultimately affirmed. Yet the opinion’s most consequential language may lie in its discussion of professional responsibility. As the court observed:  “The petitioner’s opening brief included non-existent citations and a false quotation, seemingly generated by artificial intelligence.”

After addressing the merits, the Seventh Circuit turned pointedly to the briefing:

One more issue. In its standard of review section, Dec’s opening brief cites two cases that do not exist and recounts an imaginary quotation. Such errors have tell-tale signs of AI hallucinations, in which an AI large-language model generates a fictional or inaccurate output….

When asked at oral argument, Dec’s counsel could not explain these citations. She also denied using AI to prepare the briefs. When pressed, she apologized, but she could not name the source of the errors.

In a post-argument letter, Dec’s counsel reiterated that she did not use AI, nor would she even “know where to start” because she is “way too technologically challenged to even attempt to use AI.” Instead, she “presumably copied and pasted that portion from another brief several months ago,” but she has not “been able to locate it.” She “regret[s]” not double checking for accuracy. As one who has practiced law for over ten years and held positions in the government for over fifteen, she acknowledged the need to be more “scrupulous.” She also took responsibility, stating, “This was clearly my error.”

Id. at *5 (emphasis added).  The court added:

Dec’s counsel ultimately assured us she “did not mean to engage in deception” or to seek “tactical advantage.” She also “sincerely apologize[d] to both the court and [her] opponent for [her] oversight.” When she prepared the brief, her firm had “dramatically expand[ed]” its caseload due to the “increased immigration enforcement in Chicago.” She also made known that, going forward, she did not plan to use AI and proposed ways to mitigate the risks of these errors reoccurring.

Id. at *5.

The appellate court wrote that “our concern lies with trained lawyers failing to check the accuracy of legal citations and quotations in their filings. We remind counsel that this is easier now than ever. Entire briefs can be submitted to Westlaw or LexisNexis to check citations.”  Id. at *5.

The Seventh Circuit cited Fed.R.App.P. 38 and Fed.R.Civ.P. 11(b)(2), (3): “This opinion should be read to impress upon her and the bar the importance of honesty and vigilance in checking the accuracy of all citations and quotes.”

The court’s statement about opposing counsel may be dicta:  “That opposing counsel also failed to catch these errors and bring them to our attention also gives us pause, albeit to a lesser degree.” Id. at *5 (emphasis added).

While arguably dicta, this decision raises a practical question: is silence in the face of obvious citation errors becoming professionally risky?

A recent California decision highlights the need for added diligence. Litigant Forfeited Claim that Court Erred by Relying on Hallucinated/Fictional Cases (Mar. 11, 2025).  In that case, Ms. Munoz’s attorney wrote to Mr. Torres’ attorney, citing hallucinated cases. After a hearing, the court directed Mr. Torres’s attorney to submit a proposed order.  He did, and the court approved the order, yet no one checked the cases.  Mr. Torres appealed.  He argued that a decision citing fictitious authority is arbitrary and capricious.  The appellate court wrote: “We agree the court so erred, but we conclude Torres forfeited this claim by drafting and submitting the very order he now challenges and failing to object or alert the court that the cited authorities were fake.”  The California court emphasized that it is “imperative for both the court and the parties to verify that the citations in all orders are genuine and truly stand for the propositions cited.”

Taken together, these decisions stop short of imposing a formal duty on opposing counsel to audit an adversary’s citations.  Unless opposing counsel cited the hallucinated cases in a signed response, it does not appear that either Fed.R.Civ.P. 11 or Fed.R.Civ.P. 26(g) would support sanctions against an opposing counsel who failed to uncover and report an opponent’s fictitious citations.  That said, given the rising issue of citations to fictitious cases, it may be prudent to check an opponent’s citations.

But the trajectory is clear: Courts are no longer treating citation errors as purely the responsibility of the referencing attorney. At minimum, they are signaling an expectation of shared vigilance. And, now, at least two cases imply that an opponent may have a duty to check citations in the other party’s papers.

This blog was initially posted on  Electronic Discovery Reference Model and  JD Supra.