Offside “after” a goal is scored? I know, strange title. Here is the scenario. Player A takes a shot on goal while Player B is in an offside position. The ball is on frame and appears to enter the goal and completely cross the goal-line when Player B heads the ball the rest of the way into the back of the net.

Goalkeeper nor any defenders reacted in any way to Player B so it appears that he did not affect the play. Since a goal was already scored when player B played the ball, is offside called?

In this case, Player A and B have names: Christiano Ronaldo and Nani.
You can see a clip of the play here:

USSF answer (November 20 2010):
We cannot debate the results of a referee’s decision-making process at this level. That is a matter to be resolved between the referee and his/her match inspector.

No matter how it may look to us or the players, a goal is not scored until the referee says it is scored. There was a similar occurrence earlier this year at the World Cup, when the ball kicked by Frank Lampard of England clearly bounced well inside the goal and was then swept out by the German goalkeeper. We all know it was a goal, but if the referee disagrees, life is hard.

Lampard vs. Germany

With those conditions stipulated, we can say with a high degree of assurance that, if the contact with the ball is not made until after the ball has entirely crossed the goal line into the net and if there is no issue of interfering with an opponent prior to the ball entering the net, there cannot be an offside violation. In short, there is no offside violation after a goal is scored.