Semantics

With the right roll, it's amazing what players can get away with on a bluff check. If a DM is smart, he'll take tons of things into consideration during the check, like the NPC's disposition toward the players, the situation, and most of all, the overall quality of the lie.

If, however, a DM doesn't pay attention to any of these things, then expect the players to verbally stomp over everything he comes up with. Remember, the players are not always the clever ones.