It is my opinion that the DM should almost never take a turn away from their players as punishment. A failure should always result in a choice between two or more less than ideal options.
Here is a critical fumbles table which tries to take this principle into account.
- You put yourself off balance. You can either release your weapon to stay upright, or fall prone.
- A strap from your pack tangles around your arm. Attacks are made at a disadvantage until you make a successful dex save (easy).
- Your opponent parries your weapon and makes to body-check you. Allow the opponent to deal unarmed attack damage to you or drop your weapon.
- You get sand in your eye. You make attack rolls and perception based checks at disadvantage for 30 seconds unless you make a successful con save (easy). If you attempt the save and fail, you are blinded for 1 minute.
- You slip on some rubble and are in a bad stance. You suffer disadvantage on all attack rolls until the end of your next turn, or until you move five feet in any direction.
- You roll your ankle. You are slowed until you take a brief rest or until you receive one or more points of healing.
- You feel a pang of empathy towards your foe. Every subsequent attack against that enemy causes you to feel slightly uncomfortable about this whole situation.
- You give away your secret triple-bluff fighting technique! Further attacks using the same weapon against that enemy are made at disadvantage.
- One of the ties or straps on your armour gets snagged on your enemy. Moving away from that enemy causes part of your armour to come undone, decreasing your AC by 1, unless you make a successful dex save (easy).
- Your guts make a horrid rumbling. Moving more than five feet in a round causes you to crap your pants. This effect is resolved when you… well… you know.