Yoda says in ESB "Remember your failure in the forest"
Firstly, why does he fail?
Secondly, when he chops "vaders" head off why does it show lukes face?

he failed because he let his emotions take over, his hatred clouded his mind.

It's symbolism, that if luke doesn't change fast, he'll become what he hates (basically another Darth Vader-ish person)

PS: He says failure at the cave, not forest. :p

Exactly right.

Luke's face inside the helmet can also be seen as foreshadowing of his revelation at Cloud City...

Originally posted by Kurgan
Exactly right.

yay! go me.
Originally posted by Kurgan
Luke's face inside the helmet can also be seen as foreshadowing of his revelation at Cloud City...
indeed. Another thing was in RoTJ when he cuts off vaders hand, he sees his hand and vaders mechanical hand, and realizes he mustn't continue letting his anger take over, and so he throws his saber away. :)
I thought that was a powerful moment too.

Grazie.
And yes it was the cave, small mistake on my part.
It all makes pefect sense now.