V belts slip, example: the squeaking of inadequately tight belts ,
like you hear on power steering pumps doing, in your car when turning the wheels, paralel parking.

toothed chainrings. and roller chains skip as the roller is pulled
off an excessively worn match of tooth and roller spacing..

the current trend to make click shifting work so early,
makes the situation worse, sooner
as the teeth have been shortened and shaped to make the shifting,
derailing, on & off the cogs easier..