offside -
(sport) the mistake of occupying an illegal position on the playing field (in football, soccer, ice hockey, field hockey, etc.)
mistake,
error,
fault a wrong action attributable to bad judgment or ignorance or inattention; "he made a bad mistake"; "she was quick to point out my errors"; "I could understand his English in spite of his grammatical faults"
Adjective
offside -
illegally beyond a prescribed line or area or ahead of the ball or puck; "the touchdown was nullified because the left tackle was offside"