Spell failure

From Talus Wiki

Spell Failure: If you ever try to cast a spell in conditions where the characteristics of the spell cannot be made to conform, the casting fails and the spell is wasted.

Spells also fail if your concentration is broken and might fail if you’re wearing armor while casting a spell with somatic components.

Spell failure may occur while the caster is under the effects of a taunt or the Silence spell.