So if the player receiving the ball is behind the player playing it, he's offside. I'm not entirely sure I've ever seen that given. I should pay more attention.
Hopefully this explains what I mean. Player 1 crosses the ball (red line) towards player 2. Everyone misses the ball it goes across the penalty area untouched. Player 3 who was in an offside position when the ball was played by player 1, then runs back to collect the lose ball. This would be offside. Obviously a rare enough situation but it is possible when the ball is played "backwards".