Do not get complacent about a latch that no longer goes all the way back or a door that sticks when opening. One day you might need to get through that doorway quickly. Examine stall walls for changes over time. Boards can rot and weaken. Fighting horses can loosen the partition between them.
Nails can work their way out, or a chewer can eat down the wood around a nail.
Nails and other sharp objects can cause lacerations. Nails are particularly a problem with temporary stabling. No matter how nice the facility, your stall might have been used as a tack stall. Back home, your stall flooring needs to promote drainage, otherwise bacteria could flourish, leading to thrush or pneumonia. A formerly level, or properly sloped, floor can change as the horse paws under the feed tub, stamps at the door for dinner, or urinates repeatedly in the same place.