I've only been in the presence of someone having a seizure twice.
The second time was going before I encountered the scene and appeared to be handled, so I moved on.
The first time, however, was when I was one of the people in charge of the video gaming room at the convention I staff at, and this guy was watching a couple of others place a flashy (strobing) video game in the back of the room when he dropped and went into a full GM. Whoever had been patrolling the back of the place managed to usher people forward, tell another staffer to get the room cleared while he was moving chairs and tables out of the way. Someone at some point called the paramedics, who were on scene quickly (near LAX - great response time), and the rest of us stood outside the room and told people it was a medical emergency and we were closed until we were open again.
He'd never had a seizure before then and thankfully didn't hurt himself at all, but he was going to get himself a proper checking out by his regular doctor at the first opportunity. He said afterwards that he had no idea that anything was wrong; one minute he was watching the game and the next he was on his back looking up at the paramedics. We were just glad that it happened in a controlled environment with enough people on staff to deal with it.
Faith is about what you do. It's about aspiring to be better and nobler and kinder than you are. It's about making sacrifices for the good of others. - Dresden