"Things could be worse. Suppose your errors were counted and published every day, like those of a baseball player." - Anonymous
An Error is a mistake by a fielder that allows a batter to reach base, or a runner to advance an extra base, or allows an at bat to continue after the batter should have been put out; it is determined by the judgment of the official scorer. In most cases, a batter who reaches base as the result of an error is charged with an at bat, while any run which he eventually scores is considered unearned.
Categories of Error
Throwing errors are throws that cannot be caught by the players for which they were intended, either because they are off target, or because they hit a baserunner or umpire. A throwing error is also charged if a fielder makes an unnecessary throw that allows a runner to advance.
Also common are fielding errors which include dropped fly balls that should have been caught but are not, and ground balls or base hits that either go by a player, or are not handled cleanly, allowing a baserunner to advance.
Less common are dropped throws, where a fielder will not handle a good throw from another player, resulting in a runner being safe when he should have been put out. In this case, the fielder who made the throw is credited with an assist, despite the fact that no out was recorded. A fielder will also be charged with an error, and the thrower credited with an assist, if he catches a throw in time to record a force out but fails to tag the runner or base.
Even more rare is the catcher's interference: an error is charged to the catcher when he touches the bat while the batter is swinging at a pitch. In this case, the batter is not charged with a time at bat, but moves to first base, as if he had been hit by the pitch.
A fielder is also charged with an error if he is guilty of obstruction and the umpire awards any baserunner at least one base.
Not all misplays result in an error. Errors in judgment are not usually counted as errors. For example, if, because of confusion or miscommunication, a fly ball that is easily catchable falls between two fielders without either of them touching the ball, no error is charged, as it is impossible to determine who is the guilty party. Similarly, if a fielder chooses to attempt a difficult play at a base to try to retire a runner and fails, instead of making an easy out on the batter, the play will be scored a fielder's choice and not an error.
Some other common plays that look as though an error might be charged but it is not include:
- A wild throw attempting to prevent a stolen base, even if a good throw would have retired the runner. An error is charged only if the throw is so wild that the runner is able to advance another base.
- A wild throw attempting to complete a double play or triple play. An error is charged only if the throw is wild enough to allow any runner to advance an additional base.
- If the fielder muffs or fumbles a ball but recovers to record a force out at any base, he is not charged with an error even if perfect play would have resulted in a double play.
- No error is charged if a fielder deliberately allows a foul ball to drop in order to avoid a sacrifice fly.
- No error is charged on a wild pitch or passed ball, even if it allows the batter to reach first base safely on a dropped third strike.
- Benjamin Hoffman: "Baseball's 500,000th Error Finds Jose Reyes", The New York Times, September 17, 2012 
- Tom Ruane: "Do Some Batters Reach on Errors More Than Others ?", in The Baseball Research Journal, #34 (December 2005) SABR, Cleveland, OH, pp. 113-120.
- Major League Rules 10.13 and 10.14, which cover errors.