You'll hear an annoying squeal. Maybe eventually the battery might die on you if the belt is slipping enough. Much of it depends on what the belt is actually driving which is probably more than the fan.
Glazed or loose fan belt?
check tension on fan belt(s)
rub soap on the belt
Loose or glazed fan belt?
the squeeling noise is from the fan belt either the belt needs renewing, or the a/c compressor clutch is not operating correctly
Could also be the belt tensioner ...
loose or glazed fan belt.
It sounds like the fan-belt is slipping.
depending on how low it is, it is either the fan rubbing against the casing or the belt that turns the fan coming loose.
A loose or glazed "fan belt" is a very common cause of squealing. A quieter squeal is produced by brake linings that are almost worn through.
Usually, it is your fan belt slipping due to moister. You can get a spray for it at your local auto store.
Thee fan belt is loose or dried out. Most likely loose.