I had a small FRN Case lockback fail. I was pushing the tip into some heavy cardboard and the blade collapsed. The ricasso area hit my fingernail which stopped the blade for the most part but it did slide onto the blade some and cut into my nail as well as a shallow cut on my finger. That was about '95 just after starting high school. Surprised I remember since the rest of the time is a bit hazy.
I have had the most problems with liner and frame locks. I had a Kershaw steel frame lock move all the way to the other scale with light use in about a year. It looks almost brand new except for the.excessive vertical blade play. Had a cheaper Beretta liner lock do the same thing. I think it was also one of the ones that closed on me from squeezing the handle and putting repeated on and off pressure on the blade. That and putting torque on a blade by twisting out of a cut seems to make liner locks 'walk' towards the unlocked position. Also had a S&W SWAT knife close that way as well as a Kershaw Blackout (I think, maybe a whirlwind. Also one of the first decent knives I bought and the first knife I lost). I had another liner or frame lock close on my finger while whittling into a dead tree. It was the most recent and the only one I don't remember the model. I'm with KalEl, I don't trust or buy liner locks anymore and not real big on frame locks either. You can get good ones that will last a long time, but you can also get another of the same model and have nothing but problems. They are hit and miss and I feel there are much more reliable lock designs available and liner/frame locks are not worth the risk.
I also had 1 omega spring on a Benchmade AXIS lock break. It was on a Benchmade Rift which is my all time favorite knife design as well as my favorite lock. Some people call that a lock failure but the knife still locked up safely so I don't consider it a failure. I was able to take a piece of MIG welding wire and make a copy of the spring with 2 pairs of needle nose pliers in short order, and it has been going strong ever since. I always have one of my RIFT in my RFP every single day. They see a lot of use and they also get flipped opened and closed 10s of thousands of times to keep my hands busy while not doing anything or watching TV. It's better than smoking.
I also have a broken Kershaw RAM but I guess its not the lock and also is my fault. Because of how the lock works there is a pin in the blade tang that the lock engages and there is a channel milled into the aluminum handle slab. I dropped the knife which dented the aluminum and pushed enough material in to deform the slot the pin rides in. Because of this the blade will not open all the way because the pin hits the dented handle and stops. Not sure what to call it but not a lock failure. I guess it's just broke. Which is a shame because it is one of my favorite Kershaw designs and I also really like the design of the lock. Because the lock is to the side and not in line with the blade, they were able to make a big tang and use big washers. Since the washers support the blade further from the pivot, the RAM has essentially no blade play. The MUDD also uses the HAWK lock and is one of maybe 2 knives that I can detect zero blade play. Absolutely no movement at all. Every other folder has blade play, its just an issue of how much.
That's all the problems I can think of. Since I have already written a wall of text, which I apologize this got so long, I will go ahead and write a little more. I would call a lock failure as a knife closing unexpectedly. It doesn't even have to break. A lock is supposed to keep a blade locked open and if it doesn't do that, it is a failure in my book. Even depressing a lock back while in a tight grip and unlocking the blade so it closes is a failure as far as I'm concerned. A worn out lock is where the lock wears enough that blade play is introduced and can't be easily fixed. In my experience liner locks wear out the quickest. A broken lock is where something breaks, obviously, but the lock can still work, as with the AXIS lock and a broken omega spring.
The end. IMHO. My 27 cents worth. Etc.
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The day picks you.