Am sure there was an offside goal given at Old Trafford last season very similar, AR simply got caught ball watching, was 4/5 yards off 2nd last defender and before you know it, striker had ball and AR was in no mans land.
Should it happen? No. Are we all human, even at the highest level? yes. As the above correctly points out, it will happen to me, you and your pal, and probably in every game you could freeze frame and we be out of position at some point, it was simply that, nothing more, nothing less, he was caught out of position, same happens to a gk if he cant decide whether to come or stay to clear a through ball and so on......human
in a strange way, the AR deserves a bit of credit for not guessing, and I mean that, he must have thought, oh oh, that looks offside, but, from where he was, it would have been a guess.....if that makes sense.