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.
This isn’t my hobby printer, but I once was trying to diagnose a filament feed issue with one of the Stratasys printers I run at work. This particular machine was always touchy about filament change overs (its a 4-bay F-370) and it of course failed when changing from empty spool 1 to half full spool 2. I tried remotely re-trying the filament load but it just kept complaining, so I went out and tried to manually feed the filament multiple times
It took me like 30 minutes of this to realize the feed tube had just come undone from the back of the spool drawer and I had fed 50 meters of filament into a huge tangle under the machine…