As the comment there says, what ever happened to giving the benefit of the doubt to the attacking team in offside decisions?
If the linesman thought he actually saw Forbes in an offside position, then fair enough, honest mistake. But even Father Dougal is saying what we all suspected -- that the linesman most likely didn't see Forbes until the last minute, and ended up guessing, instead of saying "I dunno" and leaving it up to the referee to decide. And if the referee couldn't tell either, by giving the benefit of the doubt to the attacking team, the goal stands. Simples