I ran the line in a game a few years back for my old under 15's game, I'd stopped playing but used to go and watch.
Anyway last min our lot 2-1 down
We get a corner keeper goes up, they counter we have 1 player back anyway breaks forward our player is in our half edge of centre circle their player 2 yards the 'right' side of him but in our half, balls played I flagged offside, proper ref didn't give it.
Yet I was convinced at the time and remonstrated with the ref after that it was off as you have to have 2 players playing the player on so in effect our defender was the 'keeper' so their player WAS offside!!