Tell your players the hallway is perfectly safe and they won't believe you. Not until they've checked it at least 3 times. But tell them it's trapped. Oh then the fun begins. You can be entirely truthful. They still might not believe you. In any inevitability they will assume you couldn't possibly be telling the truth about something like that. Not when they haven't checked for traps yet. Maybe they'll double check. But more likely they'll blaze ahead safe in their belief that you wouldn't ever tell them something so unabashedly like that. *twang* goes the arrow trap.