Whoops. 
Took me a long time to learn that one, only it generally 
wasn't the powerful monsters. The powerful monsters more often than not slipped on a banana peel and hurled 
themselves into Mt. Doom, screaming 'a double 1 on a 
balance check?!!! Noooooo!!!' 
*Phwoomph*!
No, it would be the goblins, kobolds, and assorted minions that would get crit after crit....
Never a TPK because of it, but the number of times one or two people had to make a mad dash for town, dead companions tossed onto the pack mules....
It took a while but I learned, I roll in the open until things start looking grim, 
then start hiding the 20s.
The Auld Grump