I've never been on a plane that had an issue but I have seen a few passengers have one.
Recently coming back from Europe we first heard the call for a doctor and then when we were past England and heading out over the Atlantic the plane turned around and landed in Scotland. Full ground emergency crew on landing. Plane taxied to remote area, ambulance pulled up, paramedics entered plane and passenger walked off.
Weirdest or worst was also coming back from Europe years ago. I don't sleep well on a plane so I went back to the galley for another beer. When I stepped through the curtain there was a young lady crying on the floor and several stewardesses standing around her. They asked me to go back to my seat. After a few minutes, they brought the girl up to my section and sat her in an empty pair of seats next to me. The stewardess leaned down to me and said a guy had touched her in coach and they were moving her up to get her away. When the plane landed, the pilot asked everyone to stay in their seat while the FBI took care of something. A couple of full gear FBI agents went to the back to get the guy and I assume the highest ranking female Lufthansa representative (she looked important
) at the airport got the girl. I checked the news later and there was nothing.