(though I prefer ZZ)
I was a TA for an introductory CS class that taught C++ and, in passing, vi. A hour before one assignment was due, a student showed up in a panic. “I just had it working but then the computer corrupted my file. Look! Can I have an extension?” The other TA and I smirked: What a lame excuse! We offered some generic advice about starting earlier and visiting office hours. He left in a huff.
A few minutes later, a second student appeared with the same story, and then a third and fourth.
We eventually tracked the problem down to some handwritten notes, where someone had written a largish :x for “save and quit.” The students were doing things like spamming :X (since it didn’t seem to respond the first time—-and it was over a sluggish ssh connection) or a reflexive quit-and-compile cycle. I think we eventually recovered one or two assignments by guessing what they might have done.
We obviously apologized profusely and the next class started with a discussion of :x versus :X—-and emacs!