Multiple times, usually from my own stupidity during abusing them. Throwing knives that aren't throwing knives, mostly. using knives as a screwdriver, hammer, and other things run a close second. I don't think I've ever had any decent knives fail while using them properly although I've had a few junky cheap knives experience lock failure. Generally on about the third or fourth time in a row to make sure a liner has engaged is about the time that knife goes in a junk drawer or tackle box and a newer and better knife gets into rotation. They aren't even worth giving away, I value my friends and relatives too much for that.