Well I've just broken a tooth off an end mill when I moved in Z when I meant to go in Y. Fortunately the mark was in the workpiece rather than the table, and in an area that will be waste.
I'd define both our events as a "crash".
I seem to make most mistakes by moving at G0 speed when I mean to go G1. I break the cutter sometimes, but others I just get really big chips.
I'm hoping the stepper motors will stall if things get seriously gummed up, and before damage to the machine itself occurs. I have started to use the Z-inhibit feature in Mach3, to prevent the uninteded plunge that you describe.