Darth Vader says to Luke in the Light Sabre Duel.

"Obi-Wan has taught you well"

Considering he killed Obi-Wan two films ago films ago and in the previous film already had a fight with Luke where he beat the crap out of him and then he comes out with this line.

Bad script writing that.

Unless you believe Vader is in Contact with Obi-Wan but I don't think that at all but it is a slight possibility but I put it down to error than a hint of Vader talking to Obi-Wan or Obi-wan even showing up to speak with Vader just seems off. no no...