To prove a mutual mistake, the evidence must show that both parties were acting under the same misunderstanding of the same material fact. Id. A unilateral mistake by one party, combined with knowledge of that mistake by the other party, is equivalent to mutual mistake. Davis v. Grammer, 750 S.W.2d 766, 768 (Tex. 1988). But, "[a] mistake by only one party to an agreement, not known or induced by acts of the other party[,] will not constitute grounds for relief." Smith-Gilbard, 332 S.W.3d at 713-14 (quoting Johnson v. Snell, 504 S.W.2d 397, 399 (Tex. 1974)). Mutual mistake should not be available to avoid the results of an unhappy bargain. Williams v. Glash, 789 S.W.2d 261, 265 (Tex. 1990). Parties should be able to rely on the finality of freely bargained agreements.
New York Party Shuttle LLC v. Bilello, (Tex. App. - Houston [1st Dist.] 2013)