On the third day when Seth brings Holly water, she uncaps and drops the cap before she drinks. As Seth leaves, Holly grabs the food bag and the other bottle of water, knocking over the first bottle of water - which is now full and capped.
When Holly is kidnapped by Seth her hair is straight. When she awakens in the cage her hair has been set.
Pancuronium Bromide is safe... If you perform an endotracheal intubation and mechanic ventilation after administering it. It's an old anesthesia drug and it paralyses your breathing musculature. The character says he used it on himself. He was all alone and that would certainly kill him. The drug would certainly kill the girl also as we don't see him doing anything to aid her breathing.
When Seth is reading Holly's book the time on the clock near the night stand says pm when it should be AM.
The door of the cage seems to be secured with a cipher lock. Seth is seen unlocking it simply by fixing the right numbers. Holly had plenty of time to try all the numbers possible, even for a 4 digit lock, in the attempt to set herself free.