Term used to refer to a situation in which the mistake of one party has yielded a undesired benefit to the another party. Coined after accidental anal surgery was performed on a patient, leaving the patient with a undesired "new" anus but without the originally desired surgical correction.
"You know Marty Benson? City came to chop down his neighbors tree when he complained it was crossing the property line and they cut the wrong one down. Totally fell on his house. Boy did he ever get the anus though. City built him a new house, but he still complains about that damn tree."