Have you ever listened to a song and you can hear mistakes or some sounds that aren't supposed to be there? I love it when that happens, it's funny.
Please post examples if you know any, I know only two: (the songs are really heavy I'm just saying so you don't jump 3 meters in the air)
Primitive Man: Unable, at the very end someone starts to talk.
Meshuggah: War, he giggles after the scream at 2 minutes 10 seconds.