Only really major hell job for me that comes to mind was working at the switchboard of a large hospital that was nearby. We handled all the incoming calls to the hospital and had to route them around to what people were looking for (like "hey can you put me through to family medicine, I need to make an appointment"), and also route internal calls from the facility to either someone else internal or outside to whoever they needed to talk to. We also had to deal with sending messages to pagers (since cell signal inside the hospital was atrocious). We also dealt with paging codes, which was always usually pretty straightforward, but also stressful because any delays in response were automatically your fault until you've managed to handle the protocol for contacting everyone involved (depending on what the code was). Anyway, the day-to-day wasn't too terrible, especially on day shift, as it was very straightforward since the entire hospital was open, and practically all you had to do was just connect calls. Evening and night shift was a little different, since although the call volume dropped, the amount of work you had to do per call tended to increase, since you had to manage connecting patients to on-call doctors after hours, and other pain in the ass things. Normal customer service hassle stuff also applied (like getting cussed out by people that you can't talk back to, etc.), along with getting reamed by doctors and nurses for stupid stuff.
Day shift was actually pretty great, but evening and night shift though was where it started to suck (especially night shift). I only got to do day shift during training. All the day shift people were full-time employees and also had worked there for like 20 years, and weren't going anywhere, anytime soon (one of those jobs where you can't get the slot until someone literally dies or retires). They put me on evening shift for a bit (I was part-time so they shuffled me around, though only between evening and night) which I still can't decide if it was better or worse than nights.
Anyway, what made it a hell job for me was they primarily had me work night shift. During that time, it was only one person, just you. That isn't always so bad (I'm a pretty solitary guy anyway) but the place they had you working in might as well have been a dungeon. You were in the basement of the building, in the center of it, so there were no windows around at all. You were also under 9 stories of concrete, so cell service was completely out of the question. There was no wifi that you could access (not that I had a smartphone at the time, and you weren't allowed to bring in a laptop). You had a computer in front of you for handling all the switchboard stuff, but it had no internet (there was only a local intranet). Lastly we had a small CRT TV up in the corner of the room, but it only had local access cable, so pretty much the only thing on was infomercials at that time of night (though once in a while you got lucky).
Basically, all you had was the switchboard phone, some TV that wasn't even worth watching, and yourself. Nothing else. You had to sit there and rot, while also maybe taking, at most, 10-20 calls in an entire night (this was both good and bad). It was hell trying to stay awake, unless you just brought a bunch of energy drinks to keep in the mini fridge in the room. You couldn't get up and leave either, since it was literally just you in the room, and you had to be in there at all times in case someone called (or god forbid a code). If you needed to use the bathroom, your only option was to wait halfway through the shift, where someone from security would stop by and see if they needed to cover the switchboard while you went to go piss or whatever (they had formal training on the system, but only a minimal amount, and if you took too long and a code came in and the security guy botched it, it was still your ass). They were good dudes though, and if you absolutely had to use the bathroom or something outside of that mid-way break period, they would come down to cover, but if they had stuff going on (like someone going apeshit in the psych ward), it could be some time, since they were also on a skeleton crew at night.
So yeah, barring some exceptional nights (like the insane amount of stuff I had to do after a dude got stabbed in a bar downtown one night), it was mostly you just starting at the wall for 8 straight hours, constantly glancing at the clock and watching your own mortality tick away.
There were some small highlights that broke up the monotony from time to time though. Like one night I got to listen to radio traffic of security chasing an escaped patient all over the campus, that was pretty hilarious. I also got a call one night from the clinical coordinator (it was a duty that rotated between nurses for after-hours, where they essentially assumed acting authority over the entire hospital) who told me to page the hospital president, CEO, the chief of infectious disease, and told me to announce a lockdown. I wondered if this was the beginning of the zombie apocalypse, and that patient zero was right here at this hospital.
The pay also wasn't terrible for the work involved. My base pay was a few dollars above minimum wage, and I also made night and weekend differential to add up to a few more dollars an hour. Also despite being "part time" I still pulled in 40 hours a week, and sometimes overtime. I remember making close to 30 dollars an hour for the shift that was an overtime-night-weekend-holiday.
Still, ultimately it was slowly devouring my soul, and I'm glad I got out of there, despite the positives that did exist with it.