As a matter of fact... I have 2 people that witnessed me rolling 5 20s in a row (and a miss) for 5x normal damage on my first attack in one round, and 4 20s in a row (and a hit) for 4x normal damage with my 2nd attack in one round of combat, effectively quartering a Greater Mummy in one round of combat, solo.

The system we used:

• Roll a 20, it's a chance at x2 damage.
• Roll a miss, it's normal damage. Roll a hit, it's x2 damage. Roll a 20 again, it's x3 damage with a chance at x4.
• Roll a miss, it's x2 damage. Roll a hit, it's x3 damage. Roll a 20 again, it's x4 damage with a chance at x5.
• Roll a miss, it's x3 damage. Roll a hit, it's x4 damage. Roll a 20 again, it's x5 damage with a chance at x6.
• Roll a miss, it's x4 damage. Roll a hit, it's x5 damage. Roll a 20 again, it's x6 damage with a chance at x7.
• Roll a miss, it's x5 damage. Roll a hit, it's x6 damage. Roll a 20 again, it's x7 damage with a chance at x8.

Etc.

And I'm the only one in our D&D to group to ever get past x3 damage, let alone past x3 damage on 2 consecutive attacks.

The DM even made me change dice before the 2nd attack because he thought my dice were rigged or something— everyone was so stunned by all the 20s —and then I did the same thing for the 2nd attack.

I call that the day I used up all of my luck for the entire rest of my life. (And to this day more than 23 years later, I've never again seen that kind of luck, even being a DM for the San Diego Comic Con for a few years.)

The system used depends on the DM at the time, of course, and how they want to handle critical hits— some use dice; some use decks; some use tables.

It wasn't until 3.0 that there was an "official" critical hit system built into the game. (And, if I recall correctly, many people hated it, but everyone got used to it. Our group— we kept our old 2E system, above.)