Kid In King Arthur's Court, A - CD Player Laser
There is a scene near the end of the movie where the kid (Forgot his name, been quite a while since I last saw the movie) uses a laser beam from his portable CD player to blind an opponent. There are actually three slipups here:
Slipup 1 - The laser beam wasn't coming out of the lens where it really should be coming out of. It was coming out of the center spindle which the CD is mounted onto.
Slipup 2 - Safety switches prevent the laser from being active while the lid is open, and the only way to defeat this is to stick a pin or something through the hole and press the switch. There was no evidence of such, yet the laser was still active with the lid open.
Slipup 3 - Even if the safety switch was forced closed with a pin, the laser beam still wouldn't be visible to the naked eye. In fact, it says right on the bottom of almost every CD player I've ever seen "CAUTION! <INVISIBLE> laser radiation when open and interlocks defeated."
Copy of the movie, knowledge of how a CD player works