I had a buddy who received a panic call about some machine that he had done that had some bug that couldn't be described very well but which it was imperative had to be fixed immediately. So he gets on a plane, goes to the plant and checks in. He's sent out to the machine to check it out. Well, he operates it a few cycles and can't seem to find anything out of order. Tells the powers that be, who say they'll send out the operator to demo the problem. So the guy comes out and he presses several buttons on the control panel simultaneously, while reaching way out with his foot to trip some sensor in the machine, causing the machine to malfunction in some way. That was the "bug".
I knew another guy who was on site commissioning some equipment which was functional enough to use and needed to be. So during the day, he would do software things he could do online (the platform allows online edits without a restart), then after hours do more invasive things he couldn't do while running. So he comes in the next morning and is told the machine won't start. Goes over to the machine, presses "Start" on the touchscreen, and wham-o, magick-o it starts. Turns out the operator can't read (!) and has learned to operate the touchscreen by remembering button positions. The guy had moved the location of the start button the previous night to make room on the screen for some added features.
So anytime you think you've constructed a foolproof system, the universe simply starts supplying a better grade of fools.
On one hand you have users who can crash a system.
You haven’t been doing your job if they can do that in normal use. But even if they’re being stupid you should not let them crash it. The monkey test on the keyboard (or buttons) is a popular QA test. The meaner QA people will literally unplug your database or application server in the middle of something. But the meaner QA means less chance of a user being able to screw it up. You do have to protect them from themselves to some extent.
On the start button, good UI principles can erase a lot of user complaints, but at some point you just can’t overcome a user’s stupidity. I’ve seen someone printing over and over and over and complaining the printer’s not working. He had the Microsoft XPS writer set as his default printer. You can’t really do anything about that but try to educate them.
Except on a Mac you see a picture of the printer you’re printing to, so he may have figured out a similiar situation on a Mac by himself.