My mother used to work as an occupational therapist at the local mental hospital. When the weather was nice, they often used to take a minibus full of patients out to the local park. Of course, anxious not to lose anyone, they carefully counted the patients out and then counted them back in again.
Except that one day when they returned, they realised that they had one more person than they'd set out with. Horrified that they had somehow managed to bustle an innocent bystander onto the bus and transported him to the mental hospital, they quickly began asking everyone who they were and whether they had gone out on the bus.
To my mother's relief, it transpired that one of the patients who was allowed out by himself had felt tired and, on spotting the hospital minibus, had hopped on to get a lift home, but she said they had an anxious few minutes until he owned up. :)