Tourette syndrome (3.5e Flaw)
From D&D Wiki
Your character has Tourette syndrome.
Effect: You have physical and vocal tics. Whilst you can suppress the urges for most of the time, sometimes you must find release at inopportune moments. You take a -4 penalty to Move Silently and Balance checks. Your arcane spell failure chance for spells with somatic, verbal, or material components increases by 10%.
Roleplaying Ideas: In the medieval era, sufferers of Tourette's syndrome were sometimes believed to be possessed by the Devil. In a fantasy world where these things are real, would your character believe this too?