Came close once. We had a case of an oddball grad student from Iran, who was constantly getting into fights with other people (verbally). He had come up to my lab once, since he needed help interpreting some of the spectra that he had collected.
hen I told him to come back later on that afternoon, he started yelling "That is unacceptable! I must have these results now! Why cannot you do this now?!?"
I tried to explain several times, that I had many experiments going on right now, and that to interpret the spectra would take several hours, but he was unwilling to listen, and started spewing insults.
When I walked away, he had tried to grab my shoulder, at which point I told him to get out, and was about ready to deck him, if he went any further. He did eventually leave.
I didn't have to deal with this character again, since I had already defended my dissertation, turned in the final copies, and was simply biding my time before I could begin my postdoctoral fellowship elsewhere.
Interestingly enough, though, a couple months after I had left, I found out that he had been thrown out of the program, and kicked out of the school entirely.
His advisor grew sick of him, and told him to transfer to a different lab. One day before he was supposed to leave, this character tells his only "friend" (more like the really quiet guy who always agreed for the sake of being non-confrontational) this: "Do not come into work tomorrow! I am going to do very bad things to the lab!"
The quiet guy then freaked, and told the senior grad student in that lab, and she told the nearest prof about the situation. The prof chews out this guy, and tells him that he'll be reported to the police for this crap.
Well, this rather belligerant fellow then goes up to his "friend," and says "(Name withheld)! I am going to kill you! You told on me!" At that time, the quiet guy starts stammering "I... I... I... didn't do it! Uh, uh, uh, (insert lady's name) did it, not me!"
So, this guy goes up to the senior grad student and says: "(insert lady's name)! I am going to kill you! You told on me!"
At that point, she immediately runs off, and gets the campus police involved, who takes away his keys right then and there, and escorting him away. I'm surprised they didn't toss him behind bars...
Last I heard, this loser of a fellow had transferred to some college in South Dakota, and I really pity the department that took him in...