Size Matters - S4-E5
Corrected entry: In episode 3-5: "Sight Unseen," the sisters learned the importance of taking mundane precautions when it comes to their personal safety, after Prue was stalked and almost killed by a mortal human being. Particularly stressed was the need to lock their door. In this episode, however, Paige comments that the front door was unlocked, and Leo says that the front door is usually left that way since otherwise demons just break the door down.
Correction: It's possible that, in the year since Prue was stalked, they realised that locking the door was more hassle than it was worth since, as Leo points out, it did keep getting broken. Maybe they changed their minds - it does happen.
Shay