In Hull v. Freedman, 383 S.W.2d 236, 239 (Tex. Civ. App.-Fort Worth 1964, writ ref'd. n.r.e.), royalties were based on the number of barrels produced per day. Hull was overpaid when production slipped below the stated threshold. However, the payor was allowed recovery because of an unintentional mistake of fact. Id. at 239. In Hull, the payor was initially mistaken about the drop in production. Id. Hull illustrates a true mistake of fact because the payor was unaware of the production drop. The parties were fully cognizant of the formula based upon the assignments and the mistake was clearly not one of law.