I haven't run into any bugs with the music player. I had to reassign album art, but that was because many songs didn't have it to begin with.
I would first try just restarting the phone and see if it fixes it. I would then try unmounting the SD card and putting it back in (sort of a reset for the card). Then I would try pulling all music off the phone and letting the phone clear its memory on the music player (possibly unmounting the card again) and then try putting the music back on and see if that fixes it. Last I would use Album Art grabber and/or iTag to put things back to normal.