26) Leaking Nitrogen
At least this sign is straight to the point. It warns people not to get on an elevator with a nitrogen tank, because if it leaks, you will die. Creepy signs like this one might save a life or two. At least someone is waiting for the tank on the other side (via Bored Panda). You do have to wonder if there is perhaps a more intelligent way to transport these nitrogen tanks that cuts out the possibility of unsuspecting people getting on the elevator with them, however.