I'd better contribute here...... in my short Machinist life, I've made a few cockups that would probably make most professional machinists cringe. Personally, I like to call them projects, but that's a discussion for another day....
Everyone's thrown a chuck key, haven't they? Well, actually, no. I haven't. I DID throw a parallel out of a chuck, however.... my lathe only does 670rpm flat out, predictably it was set to this when I forgot to remove the parallel I'd been using in an attempt to load some work in the chuck jaws square. The result was inevitable and predictable. I was extremely fortunate that the parallel flew out of the backside of the lathe, through a cardboard box and embedded itself in another cardboard box. Had it come out of the front, it would have killed me stone dead - or at the very best greviously injured me. That's easily my most scary accident to date...
Other small-fry: Forgetting to take the spanner off the drawbar & starting the mill.... got a bruised wrist from that one. John Bogstandard showed me a neat way to avoid a nasty impact: Use a ratchet spanner, the don't get the same speed up as they start ratcheting instead. Of course, the best way is to take the bl**dy spanner off in the first place!
I somehow avoided smashing a cutter up when the tiliting table I was using unexpectedly tilted; and there was that time on the sump when I accidentally took 3mm off it instead of .03mm, thanks to not tightening the tool up in the mill. Note to self - ensure any nuts/bolts/etc. that need to be tight actually are tight, especially if you think you did them up yesterday!
When using a 3-jaw chuck in a 4-jaw chuck, make sure the 4-jaw chuck is adequately tightened. Losing the 3-jaw chuck causes all kinds of interesting things to happen to the lathe, and I'm fairly sure none of them are good.
While we're back at the lathe, don't engage the screw cutting gear when you meant to engage the sliding feed, especially if you already forgot to change the gearbox from facing mode to sliding mode and smashed the tool into the workpiece in entirely the wrong direction. The ensuing tool-crash will wreck your stock, your tool, and your trousers.
That's all for now!