Simple Fumbles
Whenever you roll a 1 on an attack roll you make, you leave yourself open to counter-attack!
Each creature within reach of you can spend their reaction to make an Attack of Opportunity against you for your blunder.
Easy, simple, and maybe even an unintended blessing since you could probably retreat without any risk of AoO's after the fact.
Convoluted Fumbles
This fumble chart probably exists for the sake of if you ever run into an encounter in a realm that is truly unlucky, like in the presence of a trickster god or the Plane of Chaos.
Whenever you roll a 1 on an ability check, attack roll, or saving throw, you must immediately roll a d100 and check the Fumble Table for the result.
Excruciatingly Convoluted
This table is probably the worst thing I've ever written to include in a D&D5e based homebrew rulechange.
It will never see play.
Fumble Table
| d100 | Result |
|---|---|
| 1-9 | Drop weapon within reach |
| 10-17 | ^ 10 feet away |
| 18-23 | ^ 20 feet away |
| 24-27 | ^ 30 feet away |
| 28-37 | You fall prone |
| 38-45 | ^ And drop weapon within reach |
| 46-51 | ^ And drop weapon 10 feet away |
| 52-55 | ^ And drop weapon 20 feet away |
| 56-57 | ^ And drop weapon 30 feet away |
| 58-62 | Stumble, Can't Do Anything Next Turn (No Actions, Bonus Actions, Movement) |
| 63-68 | Stumble, All Ability Checks and Attack Rolls have Disadvantage next Round |
| 69-73 | Stumble, Enemy targets that can reach you can use their reactions to Attack of Opportunity you |
| 74-78 | Stumble, You hit yourself for Average Damage |
| 79-84 | Stumble, Next Attack Roll against you automatically hits. (Roll a d20 to check for Critical Hit) |
| 85-88 | Stumble, All Attack Rolls against you automatically hit until the start of your next turn. (Roll d20s to check for Critical Hits) |
| 89-93 | Stumble, Next Attack Roll you make Automatically misses |
| 94-96 | Stumble, You are unable to Attack any more this Round and your Next Turn. |
| 97-98 | Stumble, One enemy target can spend their reaction to Attack of Opportunity you except it critically hits. |
| 99 | Your Weapon Breaks. If not applicable (i.e. not having a weapon in hand or using one), treat this roll as a 98 |
| 100 | You automatically hit an ally within 10 feet of you. (Roll d20 to check for critical hit). If not applicable (i.e. not rolling to attack, treat this roll as a 98) |