Continuity mistake: When David and Saavik approach Spock's tube on Genesis, you can see quite a bit of dirt covering the words' red lettering, but when it does another shot of the lettering, it is noticeably cleaner. (00:30:25)
Continuity mistake: In the scene early in the movie where Kirk is talking to Sarek the guns on the wall are suddenly rearranged. (00:23:50 - 00:24:45)
Revealing mistake: When the Enterprise explodes, some of the debris briefly turns transparent. (01:12:15)
Chosen answer: Scotty spent the best years of his career as the chief engineer of the flagship of the Federation: the Enterprise. His time has passed, however and the Federation is rumored to be planning on the Excelsior becoming the new flagship. It is a larger ship with a new, experimental warp drive. Scotty hates the Excelsior primarily due to his pride in the Enterprise but also because he is unimpressed by the design of the Excelsior. The trans-warp drive is easily sabotaged.