I can't tell you how many times I've come home to find random cats that I don't know hanging around my house while my own cat is out doing whatever it is cats do. They waltz in through my pet door and make themselves at home, even helping themselves to my cat's Friskies.
That's why I'm considering the SureFlap, a secure pet door that only opens for the pet or pets that have approved RFID chips implanted under their furry skin. Any other animals, say the evil gang that likes to get its fur all over my couch, would be turned away.
This is a pretty high-tech solution for a low-tech problem, but if it's a situation you're in, then it's a perfect answer.
(Via Geek Alerts)