If you'd like to give someone a good scare and have yourself a good laugh, then you should download a new free jailbreak tweak called ScareYou by iOS developer va2ron1.
ScareYou allows you to assign an Activator action to enabling a timer that will display a scary face on the screen and play a scary sound after the timer completes. The timer is configurable, so you can time it perfectly, giving you enough time to enter your passcode and then hand your device to someone else to use. As passcode is not necessary, however, to use this tweak.
There are three different kinds of faces, as you can