Not while on a plane, but while I was walking through an airport once, I witnessed two people fall about 100 feet from each other, and 15 minutes apart.
The first was a middle aged woman who had a syncopal episode. She just kind of toppled over while walking off the moving sidewalk and made a dramatic screech. People ran over and encircled her, but no one knew what to do.
I made my way through the circle, and by this time the woman was awake, A/O. She was tearful and stated that she had just lost her mother and hadn't eaten all day. I did a quick neuro exam, checked her pulse, made her sit up against the wall, and ran to get her an $8 orange juice.
She began to drink, thanked me, then managed to walk over to the food court with her family.
As I started on my way across the food court, ANOTHER lady (this one was much older), slipped and fell. SHe didn't speak english so I tried my best to communicate with her. This time, I was concerned for a hip fx. so I found security and had them call EMS. I stayed with her another 30 minutes, supporting her head and holding her hand on the floor of a busy airport until they arrived.