Scotty illegally beaming aboard the ship (which he was never assigned to) and becoming Chief Engineer of it is even worse. Where was the actually assigned CE? Why was Scotty more qualified than that CE?
And Chekov switches department and job. Scotty didn't have a replacement? Who replaced Chekov? Is Chekov's whiz so wasted in his Navigator duties that everyone can replace him? But he is the only one able to replace Scotty in engineering?