I’ll start off with a recent one. I forgot to connect the hotend cooling fan when remounting the hotend. After a while the printer jammed and the whole toolhead got so hot that it started to melt. The brass inserts were coming out of their sockets and the hotend stood at an angle instead of straight down.
I also noticed that the “teflon” tube connecting the extruder to the hotend was not in fact teflon. The slowly melting ABS was blowing a bubble in that tube and it was clearly starting to melt.
It was quite a pain in the ass to fix as I did not have a second toolhead nor a second 3D printer to print a new one so I had to fix the extruder/hotend in place using 2-part glue hoping that the printer lasts long enough to print a new toolhead.
I should probably have either a thermal fuse or a smart filament sensor that notices jams as well. When your hotend cooling fan fails, or the operator forgets to connect it, the filament will jam in the heatbreak and if it goes long enough the whole hotend will be plastic-meltingly hot. A jam sensor should see it before the toolhead starts to melt.
Yes, it’s way worse. I made this mistake on a couple different glass beds of differing thicknesses, and each time, the print was so fused to the bed that it was impossible to remove without breaking the glass. One or two times, I completely shattered the glass. Another time, I went super slow and tried to get some warm IPA to wick under it, which sometimes helps prints release, and when I eventually got the print off, it still took a large chip of glass with it. It’s not even “adhesion”, it’s like the carbon fibre diffuses into the glass and creates some kind of next-level bond, almost epoxy-like lol.