I'm rewatching Star Trek II: The Wrath of Khan and noticed a major plot flaw. When Chekhov and his Captain discovered Khan on Cetialpha 5...?

If I recall the movie correctly

(it's been many years since I've seen it)

they mention that the planets had changed orbits since their last visit.

Apparently

the destruction of Cet 6

had caused Cet 5 to alter its orbit

in such a way that its orbit was nearer to Cet 6's former orbit than it was to Cet 5's former orbit.

They didn't know Cet 6 had exploded

simply because they weren't there when it happened

and, after all, there was a planet in (very roughly) the location that they expected Cet 6 to be.

Khan did mention that Kirk just dropped him and his people off, and never came back to check up on them.  

One mistake- Chekov was part of the Enterprise crew in "Space Seed". https://en.wikipedia.org/wiki/Star_Trek:_The_Origi...

If you notice Khan could not believe Chekov did not know which planet he was on.

So it seems plausible that when they up-dated the star charts the nova (I think that was what Khan said caused the change) and its affects were not noticed. He also did not have a perfect memory of all the incidents the intervening years.

I never have figured out how they would have kept track of systems that were several light years away from the nearest observatory.

I'm wrong, Chekov did not join Star trek till the second season. But is still listed as an original cast member. Sorry.

Maybe they didn't do a full survey of the entire system? Sensors and other scanning devices were not as sophisticated as they are later on in the canon, so it's very possible that they just hadn't mapped the system in many years, so they didn't know.

The better question is; how did Khan recognize Chekov? He hadn't joined the crew yet when the Enterprise first encountered the Botany Bay.

Here is a REAL plot twist...planets don't have the potential to EXPLODE...if there was a ROUGE collision with an object of some heft (large mass) and it HIT the other planet and made it crash towards the SUN...then MAYBE.

lol wut? do you have Autism?