When you counter a spell, the you can confuse the thwarted caster.

Prerequisite(s): Cha 13, Improved Counterspell, 1st-level arcane trickster

Benefit: Whenever you successfully counter a spell, you may cause the caster of the countered spell to become confused for a number of rounds equal to your Charisma modifier (minimum 1). Your caster level for this effect is equal to your caster level. You may use this effect a number of times per day equal to your arcane trickster level.