tuning to each instrument in the band

[Alan Tanner]

I'll tell you another one. How about someone bear gripping a Telecatser with 9's on it till the point where their hands are sweating and trembling. Seen that one more than once also.

The same kind of problem crops up with steelers who don't hold their bar straight or have a tendency to push down when they are "getting into it". This has the same effect. Guitar perfectly in tune, but not "played" in tune.