TITLE: Proofs shown to be wrong after formalization with proof assistant
QUESTION [150 upvotes]: Are there examples of originally widely accepted proofs that were later discovered to be wrong by attempting to formalize them using a proof assistant (e.g. Coq, Agda, Lean, Isabelle, HOL, Metamath, Mizar)?

REPLY [25 votes]: I learned of the following stellar example from Lawrence Paulson.
Anthony Bordg, Yijun He, and Hanna Lachnitt have been involved in an ongoing effort to formalize quantum algorithms and results in quantum information theory in Isabelle/HOL.  You can read about their efforts here.
In the course of their project, they naturally found themselves examining one of the seminal papers in the subject, Quantum games and quantum strategies, by J. Eisert, M. Wilkens, and M. Lewenstein.  As of this writing, Google Scholar claims that this paper has nearly a thousand citations.  Bordg, He, and Lachnitt found that there was a fundamental and unfixable error in one of the main results of the paper.  They explain the details in an arXiv preprint.