Here are a couple ads I got a kick out of - the first is genuine; the second, almost but not quite.
First, a British PSA advising you to know who the person is before you actually let them in. It's cute and all, and then the last line comes along and changes EVERYTHING.
Then there's this ad. Apparently, the voice talent decided to have some fun and dub a different voice-over onto an ad for a local union. The visuals, the jaunty music and the tough city voice don't exactly blend, but they do make it much more memorable. (NSFW, by the way.)