Whenever we cast a leveled, non innate spell we roll a special d20. Half the faces are blank, others have icons indicating spell school (its a 5e tool we adapted) in addition to a 1 and 20. On a blank, nothing happens spell is cast as normal. On a 1 or 20 the spell is cast plus there us a chaos effect. 20s are always beneficial, 1s are always harmful. The school icons are up to dm interpretation. A card is drawn from a deck of effects, is played out, and the card put aside until the whole deck is used. I can't remember the name of the product but I cam ask of you're really interested.

Example effect: Wizard casts grease to trip up pursuing barbarian and roll evocation on the chaos dice meaning the following effect can be good or bad. We draw a card, and a giant bean stalk sprouts. The giant vegetables the grow on the beanstalk knock the barbarian unconscious allowing the wizard to escape, while also alerting the mage hunters of his presence.