Plug in the normal-looking thumbdrive and your system displays the message "USB Device Detected ... OH YUCK !!!" and spits the device halfway across the room.
(however if you twist the case around 180deg before/while inserting, then the drive can be used in a normal fashion.)
The trick of course is that USB-power charges up a capacitor which then discharges into a solenoid which mechanically pushes the drive out of the machine. Oh yes, and "... OH YUCK!!!" is a bogus Device-ID burned into firmware. Rotating the plug 180deg disconnects the solenoid and the fake-ID.-- FlyingToaster, Apr 09 2010 Cock_20and_20Leap_20USB_20Memory_20Stick prior version only without the prank part [xenzag, Apr 09 2010] //expensive// how so ? capacitor 50c, solenoid $1.20, fake device-id, $0.75... the look on somebody's face when their computer keeps spitting out a flashdrive... priceless.-- FlyingToaster, Apr 09 2010 I like the pure-mechanical actuation idea. It would work best, I think, with the "retractable-style" drives. Extending the drive would cock a fairly powerful mainspring; retracting the drive would leave the spring cocked so that a person who re-extended the drive wouldn't notice that excessive force was required to do so.-- supercat, Apr 09 2010 //spring// I'd thought of that first, but the cap/solenoid combination lets it be armed/triggered over and over without repeated prankster intervention, powered by the USB bus.-- FlyingToaster, Apr 09 2010 This is actually a very clever idea. Any prying snoops would assume the thumb drive is corrupted or unreadable. Then on the other side you could store all your million dollar -waiting to be patented- half baked ideas.-- metarinka, Apr 09 2010 I assume it was somebody that was mauled by a wonky USB device when they were younger.-- FlyingToaster, Apr 10 2010 Prank toaster: You put in the toast, and after a while you hear "pop", the two toasted peaces of bread are left in place and the toaster goes flying.-- pashute, Apr 11 2010 How about simply one that won't bloody insert in the first place? Actually I think I've had a few of those already-- simonj, Apr 16 2010 random, halfbakery