I’ve often heard complaints about Khan remembering Chekov when Space Seed occurred before Chekov joined the crew. However, it didn’t. Although Space Seed is a Season 1 episode and Catspaw is a Season 2 episode, Catspaw has the earlier stardate, and Chekov appears in that episode. Notably, Chekov does not appear at the navigator’s station in Catspaw. He mans the science station in Spock’s absence and performs some engineering tasks. So it’s reasonable to suppose that Catspaw and Space Seed occurred before Chekov joined the regular bridge crew, but after he joined the Enterprise crew. Given that he showed proficiency in engineering tasks, he may have been on the engineering deck when Khan seized control of it. Chekov being Chekov, he likely would have tried something heroic and stupid, resulting in Khan learning his name and remembering him.