I've just finished watching the 'Space Seed' and its been ages since I last saw it. We all know that the movie 'Wrath of Khan' was a sequel to this particular episode. Generally speaking it is a good episode and well worth watching over and over.
It may have been a good idea if the makers of the movie watched the episode also, then they would not have made a big error.
In the movie Khan says 'I never forget a face' (or something similar) when he looks at Chekov. How could he remember Chekov when he wasn't even in that episode or any of season 1. ( I will go through the whole of that season later to check it out, but I'm pretty sure that is correct).
Either way for a genetic superman like Khan he comes across a little dense remembering a face that wasn't there!