Okay, I'll share mine. Not my embarassment, but a friend's and I was the instrument of it.
My friend is a Class 2 FFL (NFA-toy builder) in an NFA-friendly state, and I had made arrangements to check out some of the equipment from his private collection on his backyard range.
The 1928 Thompson? Rock and roll, baby, even with the drum.
The M1 Thompson? Ditto. Even without 50 rounds of drum-dumping joy, still good-to-go.
The AK47, built from scratch by his own two hands? Not a hitch, even though it does bang up my shoulder pretty good.
Finally we come to the M16, built by my gracious host's own two hands also, me all "oohRAH!" and "H-- yeah!" warface ready to go Baghdad on those empty milkjug "tangos" downrange, and...
My gentle friends, there is no sound quite so loud as when you're all set to unleash a nice, satisfying burst of full-auto goodness down the range and instead of that nice loud "B-B-B-B-B-BANG!", all you hear is...
just a quiet little "Click."
Re-cycle the weapon, drop the mag, check and clear, try it again.
"Click."
My friend opens 'er up right there, and sees the problem right away. "Oh. A broken firing pin."
Since the shop's a mile back up the road or so, and I have to be going soon anyway, we just close out playing with the Chicago pianos and talking shop.
The poor guy...