Cruel practical joke of the day: The wrong number generator.

If you really want to screw with someone, put this Wrong Number Generator on their landline. When your victim (wife) tries to make a call, it'll screw up some digits and randomly dial someone. Better yet, it only does this 75% of the time, meaning that it'll let them dial the right number 25% of the time—which allows them to think that they've got the sloppiest fingers in the world. We thought this would be even more exciting when someone's trying to dial 911, but apparently it lets all of those calls through unmolested.