Today I was doing the final profiling of an MDF piece (square board full of holes) almost as big as my machine can do, so the clamps were dangerously close to the toolpath. It was a 10 or so minutes job, and at the last profiling pass (let's say, 30 seconds before finishing the job) the Z plate was getting dangerously close to the clamps, so I decided to keep the EStop button at hand for the last pass (it's of the wired, portable kind).
The box slipped from my hands and fell from a laughable 15 or so cms, and when I took it up again I noticed the damn thing had triggered! the button wasn't even pressed (it didn't fall on that side). Well, in my nervous cursing I forgot to check the current g-code line and decided to just resume the thing hoping the position didn't offset at the stop. It seems, since it happened at the middle of one of the sides (long straight move), that the last g-code line was already submitted, so the machine thought is was already there and decided to take the 1/8" carbide upspiral endmill for a 17mm deep ride right through the middle of the board at 210 IPM, GASP!. Somehow the endmill managed to survive the cut (about 20cm long) even when the flutes themselves are only 10mm long! (it just destroyed the upper layer of the MDF by brute force, pulling it upwards like paper with help of the lower layer's cutting).
It seems that my EStop button is quite sensitive and the inertia from the fall (or my nervous catching attempt, not sure) moved it enough to trigger but not enough to lock down.
Luckily I had a spare board and could make another piece, so all was OK at the end, and a few lessons are learned