What would make an arc fault breaker trip on a bedroom circuit other than a ground and a neutral touching (which isn't the case - we checked all the switch boxes) or a pinched wire (staple pounded in too hard)?

We got one that trips the instant you flip on the light switch...checked all the outlets in the rooms for a loose connection and checked all the grounds and neutrals in the switch boxes...

just for the record - we didn't wire this house to begin with [Linked Image]