And by job, I mean the worst specific task you've had to do while under employment (but if you have any horrible hobbies or anything, feel free to share those).
It's a close call for me.
When I washed dishes at a restaurant, I was asked to go out into the yard and clean the bins. We had about twenty wheelie bins stored outside, in various stages of being filled with crap. I was given a hose, a bottle of neat bleach and a broom and told to get on with it. The stench was hideous. I have quite a strong stomach, but when waste has been left to fester and harden inside a bin for months, even I was retching. Half way through, the chefs came out and saw the horrible filth was now running down the yard and blocking the drains, so I had to reach in my hand (no gloves or anything, we didn't have them) and unclog them. Absolutely foul. If I wasn't fifteen and shy and terrified, I would have told them to shove it. As it was, it was fucking miserable.
The other one wasn't disgusting, but was mind-numbingly dull. I was doing work experience at a magazine company, and they were lining up an annual publication they did, a guide to Welsh conference centres. They wanted to confirm that the details from last year's guide were the same, so I sat for about four days, methodically ringing up each one and asking the same set of questions. The most soul destroying was the exchange that happened a million times - 'What is your maximum hall capacity?' 'Well, it depends how the seating is arranged.' 'Whichever fits the most people'. 'Well, horeshoe we can do 60.' 'So, 60?' 'Well, lecture style we can do 150'. 'WELL WHY DIDN'T YOU TELL ME THAT WHEN I ASKED FOR THE MAXIMUM NUMBER OF PEOPLE YOU CAN FIT'.