MISSING A PITCH CALL BY A FOOT. This article by Jason Epstein identifies the worst ball and strike call by an umpire in the 2014 regular season. The pitch was a strike one to Elvis Andrus in August which was 12.01 inches off the plate. The umpire was a veteran umpire who was accurate 85.86% of the time, which is a good figure for a major league umpire.
The article also links to a BloombergBusinessweek site which provides the worst call of the previous night. For October 16, the worst call was a strike on Pedro Sandoval on a pitch that missed the strike zone by 4.68 inches. The miss was by an umpire who was the 5th most accurate umpire in 2014. The accuracy records for the 2014 season is available at the site.
Of course, I am very much in favor of the site.