I often run prints overnight and a couple nights ago was no different, but when I came into my room in the morning I smelled burning and smoke. This is what I found -
The wires (all of them to some extent) on the heated bed had finally failed (red was fully broken and the other three all had exposed wire where the insulation had rubbed away). I don’t know how or why it had been working up until then! And I’m not sure what would happen if the bare wires had touched each other.
Luckily the print wasn’t that large, so the extruder stopped before the hot end got totally engulfed in PLA. For now I’m just out a hot end, and had to air out the room of all the smoke and smell. But if I had been running a large print things could have turned out much worse.
There is no strain relief on my heated bed - the wires just stuck out without even the outer insulation protecting them. Bend anything back and forth enough and it will break.
I was able to resolder the wires to the pads and will add a strain relief (plenty I found on the forum to help protect it), but I seriously advise everyone to check their headed beds. Fix it before it becomes a danger.
PS - the blob was stuck to the hot end - I pulled it off to see if the hot end was salvageable. It’s not.