I'll start:
I was very excited because I had just thought up a great, easy, quick-shooting valve. I made schematics, drew plans, almost began making it...and I realized it looked familiar. I looked around this site and realized it was a crude piston valve

My friend, who is kind of interested in air cannon, but doesn't really think things through, told me about a great idea he had for a multiple-chamber rifle that would have great RoF...He doesn't usually get worked up over nothing, so I thought he might actually have something, until he showed me the schematics...it was essentially a revolver with CO2 gas chambers instead of bullets... I tried hard not to laugh...
I was fairly confident about my 2nd combustion, and it just wasn't working, click, nothing, click, nothing, clickBOOOOOMM.....It was the loudest thing I've ever heard...I now wear industrial ear muffs when using it.