As mentioned in Star Trek II: The Wrath of Khan, one mystery is Khan's recognition of Pavel Chekov. One infamous fan explanation for the meeting between Khan Noonian Singh and the newly transferred Ensign Pavel Chekov runs something like this:
Khan Noonian Singh gets up from his diagnostic bed where he has been reading from the computer screen. He goes to the head and pounds impatiently on the door, denting it badly.
KHAN: What's going on in there? I've been waiting for an hour!
CHEKOV: Wait! I'm not finished. Nurse Chapel says the wodka I had at my going away party
on the Potemkin makes me constipated and so she gave me a hypospray...
Khan paces. Strange sounds are heard. After a few minutes the door whooshes partway open, but sticks because of the dent. Khan impatiently forces it all the way open, and young ensign Chekov emerges.
Khan's face darkens as he smells the indelicate odor wafting from the head. He grabs the departing Chekov by the arm and shakes him.
KHAN: You!! Your face I will never forget!