Incidentally, my top tip if this happens to you:
- speak very calmly and politely
- get out a notebook and a pen
- ask them which department they work for, and write that down.

You don't have to do anything *with* that information, it just puts the shits up 'em. https://twitter.com/avoiding_bears/status/1300893071517908992

And yes, I did nick this from Hot Fuzz.

have also been known to get up in a prof's face and say 'you will NEVER disrespect me in front of my students again' but that one is for egregious Scene-makers only

not only did he never storm in early and fulminating again, he didnt actually show up on time for his seminar for the rest of the semester, it was quite striking