This probably isn't as severe as the other stories on here, but I occasionally get stress-related migraines. Sometimes--most of the time, even--I can soldier through it and get things done. If I feel it beginning to come on, I can take something for it, and it's not that bad. If I wake up with one? Well, that's another matter entirely.
One morning, I woke up to a feeling that my head was trying to hatch, and I couldn't see out of my left eye. I had been to the doctor for it, and I know it's a harmless (if you have nowhere to drive!) effect called an "aura", though I sometimes call them "floaters" because they're blind spots that pass diagonally across my field of vision over the course of about an hour or so. Sometimes this is my warning sign, because it can arrive before any pain. That morning, the migraine also affected my stomach, as is often the case. Unable to see well enough to drive safely, unable to walk without weaving, and unable to stray far from the bathroom because of the nausea, I called out sick.
The place where I used to work was a 24/7 gas station and convenience store, and it is almost always busy. The call-out policy is as follows; arrange with someone to switch with you at least a day before you're going to be out, or you better come in. This isn't always possible, and people would come in to work (and handle FOOD!) whether they were sick or not. Also, 99% of the time a request to switch would be met with, "Sorry, I have plans."
I was told, "Well, we need you here, so I'll see you at such-and-such." I don't remember exactly how the exchange went, but I didn't go in, got written up, and ended up having to bring in a doctor's note. I do understand that they were short-staffed that day, but I doubt they would have appreciated having a customer complain to them that the cashier got sick in front of them. And I was not going to risk driving when I could hardly see.