On one particular flight that I took years ago, a youngish looking guy next to me went into the bathroom before take-off and stayed in there, during take-off, and for about 2 hours after.

It was a four-hour flight, and the whole time that he was in there, my only thought was if I should say anything or not. It was post-9/11, and the fact that no one noticed that the bathroom was occupied the whole time was odd. But, I was 21, stupid, and of the "I don't want to make a scene if he has stomach issues" mentality. The main reason that I didn't say anything was that he looked like he was a spring breaker going back to college and was simply sick to his stomach.

I finally came to my senses and said something to the FA who, with others, immediately banged on the door and luckily, the guy, white as a sheet, opened it. After that, he was taken to the back of the plane and was the first one to be let off when he landed (he was led out by police escorts). I overheard one of the FAs say that he was coming down from being "on something".

To this day, I am kicking myself for not saying anything earlier while being completely grateful that nothing happened.