UV (even sunlight on cloudy days) or any eye damage will randomly trigger me. Rockstar-level use of sunglasses and management of other triggers has reduced my migraine frequency from a few a month to a few a year. Frovatriptan aborts most of the remaining ones that do start.
Dehydration or something else pushing my salt balance out, stress (new for me), eye damage/UV, and I start getting bright splodges in my vision. Unless the Frovatriptan stops it, I'm completely blind in about 15 minutes, and will be for a couple of hours. Sometimes headache and nausea too.
First time it happened in secondary school, I was in a science lesson. The lab tech tried to take me to the nurse's office and WHAM, she walked me right into the door frame. I had the deputy head teacher asking me if I'd taken drugs, I had an ambulance, two sets of blood tests etc. No-one said "Yeah, migraine mate, sleep it off." It was undiagnosed for years.
Worst pain of my life was a sinus infection triggering a migraine. I thought I was suffering a stroke, the whole left side of my body went numb and the pain was incredible. I was just stood there blind and frozen in the shower crying and thinking I was dying, before I managed to get out and get medical attention. Once it was clear it was a migraine, it was far less terrifying.