Sometimes magic bites back. Magical Backlash is a temporary harmful condition triggered by unstable or conflicting magic. This only occurs when a spell, magical item, enchanted zone, or specific rule explicitly states it causes backlash. If a Save Roll is a natural 20 (rolling a 1d20), the spell can accidentally affect the caster. DN decides per spell at that moment.
When you suffer Magical Backlash, you take a -1 penalty to your Constitution score (Backlash on CON doesn't stack). The duration depends on your class: Wizards recover after completing their morning studies, while Clerics must finish their morning meditation to clear the effect (see Character Builder).