In one case I was the only full time technical person in the company. This was a very early stage startup that had literally nobody else who would know how to even start reading the code, never mind find the particularly gnarly bug that was causing paid for orders to disappear which surfaced while I was on holiday. (Because I was the only person capable of writing code I could also be pretty certain the bug was one I'd introduced and missed, so I wasn't too grumpy at debugging it from the back of a moving vehicle while on holiday).

Yes, ultimately the problem there is having a bus factor of one, and we resolved that in time but in those early days sometimes there really is nobody else who can fix things.