the belt tensioner can go "soft" and let the belt jump on tickover/drive takeup

they are a bitch to change aswell

failing that, harmonic damper (crank pulley) can fail too
had a transit in on monday, it was still "in place" but when he turned the steering all the pulleys stopped and he lost steering assist, cut the belt off as the tensioner was seized and the crank pulley fell in half when i pulled the belt!!