In the first D&D tournament I ever played (1976, Tacticon II), the rule for instant kills was a natural twenty, followed by an 8 on a d8, was an instant kill.

We were in hurry-up mode, rushing down the corridor, because there were five minutes left, and we hadn't found the quest object yet. The DM said, "A Balrog fills up the hall in front of you." Don, playing the fighter in the lead said, "We keep running, through the two halves of the Balrog, What do we see?" and rolled a d20 and a d8 down the table. They stopped in front of the DM, he looked, and said, "Umm, right. You keep running between the two halves of the Balrog. The corridor turns left."

[My thief said, "I pick up his whip as we go by." We were one of the two highest scoring teams. The scores were so close, that the 2 copper pieces that whip was worth put us over the top and won the tourney.]