I was being a bit sarcastic. Obviously the refs know the rules. But it's fairly easy to get a bit wrong at the margins. I've reffed ice hockey by comparison and, while you may miss a call here and there it's pretty clear at least if you look at a slo-mo replay whether a call was right or not based on puck and skates position.

Being somewhat familiar with ice hockey cause my kid plays, and having read the description of this offsides, ice hockey is much easier because you check position of the skaters relative to the line at the same time as the puck crosses the line. My kid's league has instant offsides, so you don't have to keep track of potential offsides, but you do have to allow otherwise offsides if the defenders put the puck in.

Under IFAB law, you have to keep track of where the potentially offside player relative to other players is at the time that the ball is kicked elsewhere, which means having eyes focused in more than one place at once. Not an easy job.

Still, I was surprised when someone told me the rule was "at least two opponent players between you and the goal line" instead of only one. I had never thought of the keeper.

The player must be past the line of the ball for it to be offside too; it the ball is passed backwards or at the same line is not offside even if there are no defenders.