Had the same problem with my early prop designs. I spent so much time making the case bulletproof, and securing wires so they couldn't be pulled out or broken (these things are going to be in the middle of a simulated war, after-all), only to have the customer plug the battery in backwards (I used a AA battery holder with a 9v snap connector on it). It didn't power on at first, so he tried connecting it backwards. My props are really simple, so he was only out a LM7805, and it was easy enough for me to add a diode to my design. Never underestimate the ability of people to find ways to break your system.