Five pages of responses and this article hasn't yet been mentioned?
http://www.amateurumpire.com/others/misc/porter02.htm
I thought for sure that someone would have linked it already. It's been around for years and is a good example of how using the catcher's mitt can aid an umpire in calling balls and strikes.
Anyhow...check it out.
From a personal perspective, one pitch location that seems to be a thorn in my side is the down-the-middle, yet high, pitch that sticks the mitt. It might be right on the center of the plate, yet crosses the plate and batter at the armpits, obviously high and out of the zone. It doesn't "skim" the top of the zone and is, in fact, at least 6" out of the "midway from shoulders to waist" upper limit.
I call this a ball and inevitably recieve grief from the pitcher or catcher. But, I stick to my guns and call it a ball all game.
Anyone else have any thoughts or comments on this "down the middle, but obviously high" pitch location?