Some DVD rentals have stickers on the disk itself. I always tear those off before sticking them into my DVD player. I remember reading a bunch of stuff about how stickers make the disk unbalanced and it puts a lot of vibration and stress on the player.
when i used to work at blockbuster when someone brought back a dvd and handed it to me to check it in, i would always ask if they rewound it, just to fuck with them. some of them didnt catch on and asked if i was going to put a fee on their account...lol. the world is just full of morons