Ive had two, and they really make you relaize that you may not be as perfect as you thought, especially if you reloaded the rounds.
I handload all of my fifle bullets, and I pay particularly close attention to my hunting loads. I usually load up 10 or so premium .30-06 bullets for deer/moose, and they have always found their mark. This fall however, before hunting season, I was firing off last year's rounds to make sure all was still aligned in that particualt rifle, when one of the 9 rounds I had left, just made a pffffft and that was it. Turns out I forgot to charge the case with powder! It just makes you relize that you need to be very attentive while reloading, as mistakes can and will happen.
The other squib was on a trap line during a turkey shoot. I was shooting some old reloads I had been given, and I was half way through the shoot when I loaded up, yelled pull, and then nothing, except for the pfffft. The wad got stuck in the choke, and I couldnot for the life of me get it out. I had to forfeit the round since we were in competition It was embarassing to say the least.