I purchased a plain old 560 on the exchange about 2.5 years ago, got it for quite cheap so I was skeptical and when I got it I found out why. Major case of lock slide, the blade would push the lock bar over and close with only light pressure on the spine, no spine whacking, just very light finger pressure. I spent the cash and sent it in from Canada, as I said the price on the knife was so good that even if it cost me a couple extra bucks it was no big deal, explained the problem, filled out the form and waited.
Well the knife returned to me in good time, it was a month or less by my recollection, so no issue there at all, but the problem remained. There was a slight difference in the pressure required to "slide" the lock bar over, but basically marginal change. There had been a nice sharpening job done on the blade so that was a plus, but I was disappointed that my shipping money had been spent and the problem had not been rectified.
My solution? I took matters into my own hands and dismantled the knife, with a small tungsten file that I had I carefully re-contoured the lock bar face in a very small area to increase the contact area of the lock bar and knife tang, a minor adjustment that completely corrected the problem in less than 15 minutes work without affecting lock-up percentage in any noticeable amount. The knife locks up solid as a rock now, as it should, and it still was a pretty good deal for me.
No hard feelings toward ZT, I love a lot of there stuff, but this is just my honest story with their warranty department.