Scroll Mishaps

A creature who tries and fails to cast a spell from a spell scroll must make a DC 10 Intelligence saving throw. If the saving throw fails, roll on the Scroll Mishap table.

d6	Result
1	A surge of magical energy deals the caster 1d6 force damage per level of the spell.
2	The spell affects the caster or an ally (determined randomly) instead of the intended target or a random target nearby if the caster is the intended target.
3	The spell affects a random location within the spell’s range.
4	The spell’s effect is contrary to its normal but neither harmful nor beneficial. For instance, a fireball might produce an area of harmless cold.
5	The caster suffers a minor but bizarre effect related to the spell. Such effects last only as long as the original spell’s duration, or 1d10 minutes for spells that take effect instantaneously. For example, a fireball might cause smoke to billow from the caster’s ears for 1d10 minutes.
6	The spell activates after 1d12 hours. If the caster was the intended target, the spell usually takes effect. If the caster was not the intended target, the spell goes off in the general direction of the intended target, up to the spell’s maximum range, if the target has moved away.

Credit: Scroll Mishaps variant rule

Potion Miscibility

You can use a bonus action instead of an action to drink a potion you’re holding. If you drink one potion while still under the effects of another or pour several potions into a single container, then the strange ingredients used in creating potions can result in unpredictable interactions.

When a character mixes two potions together, the DM can roll on the Potion Miscibility table. If more than two are combined, roll again for each subsequent potion, combining the results. Unless the effects are immediately apparent, reveal them only when they become evident.

d100	Result
01	The mixture creates a magical explosion, dealing 6d10 force damage to the mixer and 1d10 force damage to each creature within 5 feet of the mixer.
02–08	The mixture becomes an ingested poison of the DM’s choice.
09–15	Both potions lose their effects.
16–25	One potion loses its effect.
26–35	Both potions work, but with their numerical effects and durations halved. A potion has no effect if it can’t be halved this way.
36–90	Both options usually work.
91–99	The numerical effects and duration of one potion are doubled. They usually work if neither potion has anything to double in this way.
00	Only one potion works, but its effect is permanent. Choose the most straightforward effect to make permanent or the one that seems the most fun. For example, a potion of healing might increase the drinker’s hit point maximum by 4, or oil of etherealness might permanently trap the user in the Ethereal Plane. At the DM’s discretion, an appropriate spell might end this lasting effect, such as dispelling magic or removing the curse.

Credit: Mixing Potions variant rule

Toxic Effects

Magical potions have powerful beneficial effects, but most beings weren’t built to handle the raw power of so much magic running through their veins. Drinking a single potion has no immediate side effects, but if you drink more before your body has the chance to clean the potion’s potent magic from your system, it could have serious consequences. 

If you consume more than one potion before finishing a long rest, you must make a Constitution saving throw after drinking it. The DC of the saving throw equals the strength of the potion you drank. If you drink three or more potions before completing a long rest, you have a disadvantage on this saving throw.

Potion Rarity	Potion Toxicity
Common	DC 9
Uncommon	DC 11
Rare	DC 13
Very Rare	DC 15
Legendary	DC 17

On a success, your body can withstand the residual magic from multiple potions mingling in your body. On a failure, the magic overpowers you. You feel queasy and vomit. You take acid damage equal to 1 + 2d8 per level of rarity above the common of the potion you drank. For example, a rare potion deals 4d8 + 1 acid damage.

Alternatively, you may be asked to roll on the Expanded Toxicity Effects table.

Expanded Toxicity Effects

d10	Effect
1	Your stomach twists and turns, visibly wriggling inside you. You become poisoned and take 1d6 acid damage per level of rarity above the ordinary of the potion you drank at the end of every hour.
2	Your veins begin to glow an unusual color, and your blood feels like it’s burning. You take necrotic damage equal to 1 + 1d8 per rarity above the common of the potion you drank. You also take this damage whenever you’re affected by a spell that deals damage. You can repeat the saving throw at the end of every day, ending the effect on yourself on success.
3	Your eyes secrete a thick, cloudy fluid veined with neon color. You become blinded. You can repeat the saving throw at the end of every day, ending the effect on yourself on success.
4	Your ears fill with a colorful, gummy substance. You become deafened. You can repeat the saving throw at the end of every day, ending the effect on yourself on success.
5	Your saliva becomes a thick, glue-like ichor, sealing your mouth shut. You can’t speak and can’t cast spells that require verbal components. You can repeat the saving throw at the end of every day, ending the effect on yourself on success. Lesser restoration also ends this effect.
6	Your legs lose all feeling and solidify into a chalky, brittle substance. At the end of every hour, you travel at a regular or fast pace overland, or at the end of every turn in which you move 30 feet or more, you take necrotic damage equal to 1 + 1d6 per rarity above ordinary of the potion you drank. You can repeat the saving throw at the end of every hour, ending the effect on yourself on success. Greater restoration also ends this effect.
7	Your skin becomes translucent and feels as thin as an insect’s wing. You take necrotic damage equal to 1 + 1d8 per rarity above the common of the potion you drank. You also take this damage whenever you take damage from a weapon attack. You can repeat the saving throw at the end of every day, ending the effect on yourself on success. Greater restoration also ends this effect.
8	Your thinking slows, and you feel a strange liquid sloshing around your skull. You take psychic damage equal to 1 + 1d8 per rarity above the common of the potion you drank. You also have a disadvantage in Wisdom, Intelligence, and Charisma saving throws. You can repeat the saving throw at the end of every day, ending the effect on yourself on success. Greater restoration also ends this effect.
9	Your limbs feel like jelly, and your flesh becomes slick and wobbles when touched. You take necrotic damage equal to 1 + 1d8 per rarity above the common of the potion you drank. You also have a disadvantage in Strength, Dexterity, and Constitution saving throws. You can repeat the saving throw at the end of every day, ending the effect on yourself on success. Greater restoration also ends this effect.
10	Your entire body convulses and then locks up. You become paralyzed until the start of your next turn. Then, you hiccough, and a tiny frog hops out of your throat.

Credit: Critical Role house rule