1st penalty decision at Anfield.

[Loramski]

Still unclear, for example if a player is standing in an offside position when a shot is taken (not interfering with play) and then that shot rebounds of the keeper and the offside player is able to score, guarantee 100% of the time that goal will not stand.

There was a similar incident in the Southampton game on Saturday (?), albeit the player missed but the flag had already gone up.

That's the exception. A save doesn't play an offside player back onside, any other deliberate playing of the ball by an opponent does.