This
http://www.baseballprospectus.com/article.php?articleid=12991
"The Brewers brought out the hate in me. I was a crazy man... I hated everything about the place. If the official scorer gave me an error, I didn't think was an error, I’d say, 'OK, here’s a real error,' and I'd throw the next ball into the stands on purpose.
Sheffield recanted that statement in a follow-up with Nightengale: "What I said was out of frustration. They want to take something and run with it. Why would a player purposely make mistakes? I'd never do anything to hurt the team. You get paid to play." There's no further evidence to suggest that Sheffield made intentional errors during his time with the Brewers, with nothing particularly damning in play-by-play accounts, and certainly nothing that was reported by observers as obvious errors. Of the four times Sheffield made two errors in a single game as a Brewer, none seem to fit his description:
April 23rd, 1989: Playing shortstop, Sheffield made a throwing error in the second inning, then another one in the fifth, although the batter reached only first base in both cases, which would indicate the ball wasn't thrown into the stands.
June 20th, 1989: Again at shortstop, Sheffield made an error in the seventh, apparently on a relay throw. In the 10th, he made an unspecified error on a ground ball, although the ball did not go into the stands, as the batter only reached first.
May 15th, 1990: Playing third base, Sheffield made a throwing error in the second inning, allowing a batter to reach first, then made another in the eighth on a ground ball, not a throw.
April 8, 1991: (Opening Day in Texas; the others were all at County Stadium): Playing third base, Sheffield made a throwing error in the third—on his first chance of the season—with the batter reaching first. He handled his next ground-ball chance cleanly, but made another throwing error in the ninth while fielding a weak grounder; again, the batter only reached first.
Nightengale's follow-up does include the following note: "Sheffield said the only time he may have made an error purposely out of anger was when he was in the Brewers' minor-league system." Which, if you look at the context of the way he was mistreated by the organization during his late teens and early twenties, makes a bit more sense. While that wouldn't excuse such an action, in the absence of actual evidence that this was anything more than an isolated temper tantrum, how much of a grudge should anyone carry? Sheffield was in a situation where he was young and foolish, and was the object of high expectations, but with little support from his employer to ensure that those expectations were met. Whatever happened, he wasn't investigated or disciplined by the team or the league, so it's not as though there was a suspicion of wrongdoing."