The two most aggravating gameplay bugs, IMHO, are this (PS3):
  1. Sliding your thumb across the direction buttons on the left side of the controller so that you hit two buttons only counts as one hit (even though YOU HIT TWO BUTTONS). (Apparently, as long as one button for the track is still pressed down, pressing other buttons which map to the same track will have no effect.... whoops!)
  2. Switching tracks while you're doing the Jackpot powerup breaks your streak, even when you don't miss any notes. (I guess this has something to do with the timing window of notes, but normal gamer-types can't be expected to understand "timing window bug" - all they know is that they didn't miss any notes but somehow lost the jackpot streak anyway.)

There's also the gameplay bug where the Road Rage car is invisible every once in a while. Since that doesn't appear to affect points scored (the invisible car still runs down the notes), I think that bug is not as serious.

This thread would be infinitely more useful (and probably have a better chance of being stickied) if the OP would summarize all bugs found on the first page - jibjqrkl!