Wizards and Priests

• Casting scrolls containing spells of a level too high for the character to cast incurs a 5% failure rate per level between the character's present level and the level at which the spell could be cast (DMG p. 196)
  • If the DM allow characters to control the casting level of scrolls they scribe, the failure chance should be 5% between the character's present level and the scrolls casting level. (Sage Advice #248 p. 20+22)
• Scroll spell failure is either Total Failure (nothing happens) or Reverse / Harmful Failure effect (DMG p. 196)

Rogues

• Thieves casting scrolls have 25% failure chance. Failure is always a reverse of the effect (PHB p. 58)
• Bards casting scrolls have 15% failure chance. Failure is always a reverse of the effect (PHB p. 61)

Dual/Multi-class Spellcaster/Rogue

• Uses the best failure chance of the two classes. (Sage Advice #260 p. 23)
  • The DM can allow the player to choose what class is trying to use the scroll. (Sage Advice #260 p. 23)
• Failure follows the rules of the class the chance belongs to. Rogues always have a harmful effect while spellcasters can have a Total Failure with no catastrophic effects. (Sage Advice #260 p. 23)