Many years ago, I was just a boy of about 12 yo, during a long flight from Bombay to Frankfurt, there were a lot of people from India on board who looked very 'traditional', quite apparently most of them had never flown before.
I was passing most of my time running around the plane, probably annoying other pax and cc....
I noticed one of the Indian pax, a middle-aged female, was trying to open one of the emergency doors. Fortunately, she was making no sense of the huge instructions in red...I remember it being quite a big aluminium handle that had to be rotated 90°...was a DC10.
I was very alarmed and found a flight attendant and told her. She apparently either did not believe me or had trouble understanding my poor English (I am German and had only started taking English lessons).
Once she saw the Indian pax at that door, she sure got to move quick!
She had to call in a male colleguage, and the 2 of them had to forcibly wrestle the woman away from the door. The FA later told me it was not dangerous, that the captain had to 'activate' the emergency doors before they could be opened, but I never believed her.
From then on, I have never stopped watching my fellow passengers a bit, just in case someone tries to do something stupid....