I'm lifting this from a post by inharmonik on IHM just because I enjoyed it so much (even better if it's actually true):

A dude was playing one of Feldman's graph pieces and it had a box with a 3 in it to indicate 3 tones. The pianist hit his three notes and Feldman grimaced..... The dude looked at Feldman like, "WTF? you said 3 pitches?" Feldman responded, "yeah, but not those three."