Floats stop working or get knocked out.
Buckets get turned over.
Buckets get pooped in.
Birds or wildlife fall in.
Many things can go wrong with your horses' water supply.
So what should you do if you find that your horses' tough is dry or contaminated?
Besides obviously cleaning and filling the trough you should withhold grain (and hay if possible) for a few hours once the trough has been filled to give your horse plenty of time to drink and re-hydrate to help prevent colic.
If you feed a horse before they have had a chance to drink for several hours the they many end up with an impaction colic from not having enough moisture to properly digest their meal.
They can also suffer from gas colic if they drink a bunch of water right before or after a large grain meal which then can give them a tummyache.
So in short to help prevent colic when #@%^ happens postpone a grain feeding for several hours or skip a feeding entirely when your horse has not had free access to water for an period of time.
This also applies when riding and trail riding for several hours at a time.