I called in fatigued once, in the middle of a duty. I was fatigued because the hotel they put me in the night before was, frankly, appalling. Fortunately, there was an FO on airport standby, he was at the steps of the aircraft by the time the last passenger was off. (Before anyone asks, the captain was not fatigued- he was locally based and had spent the previous night in his own bed). No flights were delayed, no passengers were inconvenienced...
Some weeks later, I had a phone call from crewing.
Crewing: "You never submitted a sickness self-certificate for the duty you did not complete on xxx day."
CBFO: "That's because I wasn't sick."
Crewing: "You did not complete the duty, you called in sick in the middle of it."
CBFO: "I was not sick, I was fatigued."
Crewing: "Fill in a sickness form anyway."
CBFO: "Well, if you're sure. However, the reason I was sick was, the company put me in a terrible hotel. That means, if I was sick, it was due to company action; that means it was an industrial accident. I will write that on the sickness form. Still want me to submit one?"
Crewing: "I'll get back to you. <click>"
It's been nearly three years and I haven't heard back yet!