Hm...I remember this one NPC who was like the walking library, as far as the world around the PC's went. The party listened to him for a moment, took him to their room in an inn and cut him to pieces. Just like that. They thought him a spy or some such, although I don't quite remember anymore. The point is that this NPC wouldn't have posed no threat what-so-ever to the party, and could have been a major asset. It's just that the DM didn't get the message trough before the party slaughtered him.
NPC's are dime a'dozen, if you don't bother with proper introductions, skillful portrayel and loads of character. In other words to make an NPC memorable and important enough to keep around the DM had better do his background work and NPC creation well. The best of ideas go down the tube when a party of suspicious / greedy PC's get their hands on an NPC.