rear cant really jump the track but the channel that the bottom of the window sits in can get rusty and pull off. Another likelyhood is that the the two crescent shaped gears that make up the regulator assembly may have jumped out of sync due to worn/rusty teeth on the gears. If they get a tooth off the window will get lower on one side.

Turn LEFT to go RIGHT


Thudump!....What was that daddy? Ran over a Honda, go back to sleep sweety.