I’ve put together a couple of d20 lists of critical failures you can use in your game if your players are getting a little too powerful. You can pick and choose what is appropriate for your game. As usual, these are just suggestions for a great DM to personalize accordingly.
Melee Critical Failure Table
1) You miss with a blow that is strong enough to knock the weapon out of your hand.
2) Your swing misses, but your weapon gets stuck in the creature’s armor or shield. You fall prone and have disadvantage on attack rolls with that weapon until you use an action or bonus action to free it.
3) The creature you attack moves out of the way and your attack misses, but your weapon lands where it was and you fall prone.
4) You lose your grip on your weapon and it lands at your feet.
5) You hit, but your weapon is so tangled in your opponent’s armor that you have disadvantage on the attack roll.
6) You fall down as you swing, striking out with your arms to catch your balance. If you do not make a dexterity saving throw, you also fall prone.
7) Your weapon cracks as it hits, sending a shock through your hands that causes you to drop the weapon.
8) Your miss causes you to overbalance and fall prone for 1d4 rounds.
9) Your weapon is torn from your grasp and thrown. It lands 10 feet behind you.
10) Your swing carries you around and past your target, sending you sprawling on the ground for one round.
11) Your weapon splits and the blade flies off in an arc, landing 10 feet away in a random direction. If it hits someone nearby, they must make a dexterity saving throw or take 1d4 bludgeoning damage from the shrapnel.
12) Your enemy moves out of the way, and your sword breaks as it hits the ground. You are disarmed and have disadvantage on attacks until you can find another weapon.
13) Your opponent moves out of the way, and your weapon flies out of your hand, landing 10 feet away in a random direction. If it hits someone nearby, they must make a dexterity saving throw or take 1d4 bludgeoning damage from the shrapnel.
14) The force of your swing throws you off balance, and you fall prone.
15) Your enemy’s taunts distract you, and your swing is ineffective. You lose your action for the next round as you try to gather your senses.
16) Your swing goes wide and you lose your balance, falling prone.
17) Your weapon seems to lose its momentum and falls to the ground.
18) Your weapon breaks. You can no longer use it.
19) You mean to swing at one foe, but your attack goes wide and hits another foe nearby. That creature can make an attack of opportunity against you.
20) Your weapon flies out of your hand and lands 10 feet away. If it hits someone nearby, they must make a dexterity saving throw or take 1d4 bludgeoning damage from the shrapnel.
Ranged Critical Failure Table
1) Your bowstring snaps, and your attack misses.
2) Your arrow lands in your own foot. You have disadvantage on attacks until you can make a DC 10 heal check on your foot, or until it is healed through magic.
3) Your arrow breaks and gets tangled in the bowstring for 1d4 rounds.
4) As you fire your projectile, you lose your balance and fall prone.
5) Your bowstring breaks and your projectile falls to the ground.
6) You sneeze as you’re about to fire, and you accidentally fire toward a random friendly nearby.
7) As you fire, a flock of birds suddenly flies across your aim, causing you to miss.
8) Your arrow falls short and hits a tree or other object.
9) You accidentally fire your weapon into the ground next to you, or into the air, causing your ammunition is wasted.
10) You shoot yourself in the foot. Take 1d4 damage to the foot, and disadvantage on all attacks for 1d4 rounds.
11) Your arrow gets tangled in your own bow. You lose the use of your bow for 1d4 rounds.
12) Your weapon breaks, splintering and barely able to hold together. You lose the ability to make ranged attacks until you can fix it.
13) You fire at a friendly NPC.
14) Your arrow clips your bow string, flies everywhere, and lands anywhere between 10-50 feet from where you fired.
15) As you fire, the wind suddenly picks up, knocking you off balance. You lose your balance and fall prone.
16) You fire faster than you can control, and fire into a random nearby patch of trees.
17) Your projectile gets stuck in the bow string. You have disadvantage to attack rolls until you can detangle it with a dexterity check.
18) You fire, and your projectile lands next to you, and strangely explodes showering you in shrapnel doing 2d6 damage. (Someone cursed the projectile when you weren’t looking.)
19) You attack a target, only to have another random NPC (or yourself) take the hit.
20) You fire at your enemy, but the wind suddenly changes direction and causes your projectile to sail off harmlessly into the distance.