The Cave Scene!
In the Empire Strikes Back, Luke went into a cave and encountered Vader, where at the time he thought it was real. After the defeat, Luke's face was shown underneath Vader's mask.
What Does this mean? Later in the movie, Yoda says "The cave, remember your failure at the cave" Luke "Master Yoda, I learned so much since then"
When Luke said this, he knew he did something wrong at the time.
At the time with Vader...
Was it Retaliation to destroy the enemy? (The thing a Jedi to do) or giving into anger? (Path to the darkside) perhaps due to his parents, Owen and Beru's death.
OR, was it an indication of Vader and Luke being the same person. If Luke were to kill Vader (himself) it would lead Luke along the same darkside path his father did (In another way).
The same kinda thought Luke felt as Vader was lying defeated on the death star in Ep VI.