I was running a Call of Cthulhu game once, and it was quite a long session. Two of the three players had retired for the night and the last one, whom we shall call Bob, was desperate to get to the end of a mystery that revolved around his PC and he was incredibly intense about it - which was probably why the other two players had excused themselves.
Now, Bob was desperate for a pee but I had started a dark, tension-filled scene in an old collapsed church somewhere in the south of England. He was hoping to find answers to his mystery and flatly refued to take a toilet break becasue he was just so damned excited and didn't want to miss anything! So, against my advice, we continued.
Suddenly, as he walked through the rotted pews, the floor erupted and a Hunting Horror burst from the crypt. I wanted to maximise the fright - he was mulling over a cryptic letter and as he looked up to ask a question I was loud, in his face with my arms waving. BAM! I screamed! He yelled! And he peed himself.
Just a little. It was then he decided that a toilet break was a good idea.
I shit you not.
So, in conclusion kids, the Call of Cthulhu RPG is pant-wettingly scary.