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.
I had a long print job in ABS running overnight. At some point the part detached from the bed. While the amount of spaghetti was dissapointing, the massive blob of solid ABS covering the print head was way worse. I had to cut of the majority until i could remove all non-metal parts. Then i melted the rest of in the oven.
I literally just had this happen last week but with PLA not ABS (thankfully)… I got a lot of it off and removed the toolhead (Snap Maker U1) but I haven’t had the motivation to get the blob completely off yet.