Offside, one of the most difficult things to see for an assistant referee. It’s the same for the supporters around the pitch.
Just back from my hodiday, I watched a game of the club I always played soccer myself. I got the screenshot below from a regional tv station. The blue arrow shows my position. I thought: this could have been a wrong call from the AR, so I checked the video.
It’s still difficult to see if the assistant is right (what do you think?). That shows very clearly how important the position in (non-)offside situations is. In my experiences most supporters start shouting dirty words to the AR in case of these close calls. Compliments to them: they just applauded for the striker who made a nice strike.