I believe this one occurs in the episode "Tithonus," in season 6. Near the beginning of the episode a guy gets mugged and killed. The mugger uses a knife to kill the man; afterwards, when the police are investigating the scene, they find the knife lying on the ground.

The problem: When the mugger is attacking the man, the knife he's using is a butterfly knife. Yet, it's apparently a magic knife, because the knife the police find on the ground is a switchblade!