Originally Posted by fuzz nation
By altering your grip in the middle of swinging at the ball, you were unconsciously sabotaging your own serve with something that would be virtually impossible to spot without knowing the "circumstantial evidence" that would diagnose it.
One possible way to spot it is examining the grip at the beginning of the swing and then looking at it after the end of the swing.
In my case, the Continental grip had shifted about a 1/4 inch towards the Forehand grip (not a full forehand, more like a weak Continental grip).
I was still getting a good serve despite this flaw, but it was significantly better after correcting it.
I talked to my old coach in passing and told him I have a grip shift problem and whether he thinks that's a major problem. He said he doesn't like that I do it and sees no reason that I should do it... But I didn't get a chance to discuss it in depth with him.