From Michael E. Mayeau's "The Dragon Crown" that was run at Pacific Encounters Convention in 1978 for OD&D and published the next year by Judges Guild.
After a roll of a natural '20' roll another d20:
1-14=No critical
15=Maximum damage
16=Damage roll x2
17=Maximum damage x2
18=Damage roll x attackers level
19=Maximum damage roll x attackrs level
20=Instant death
After a roll of a natural '1' roll another d20:
7-20=Normal miss
6=Stumble, roll your dexterity or less on a d20 or fall. If you fall, each melee round you may attempt to rise by rolling your AC or less on a d10 (with special things about magic armor and getting help).
5=Weapon breaks. If a magic weapon, roll 2d6 and add your weapons plusses to the roll, if your total is 7 or less your magic weapon broke.
4=Hit nearest ally for 1/2 damage
3=Hit yourself for 1/2 damage
2=Possible critical hit on nearest ally.
1=Possible critical hit on yourself.
If
@Mort thought the usual fumbles were bad for multiple attack martials, this gives you a (granted small) chance of killing yourself on each attack. Things were sure different back then!