Star Trek: 10 Times Canon Got In The Way
1. Blame The Transporter
Some might say Unification is TNG's best crossover with The Original Series but it's a concrete fact that this accolade has to go to the season six Scotty beam in, Relics.
There's a reference to every season of the classic 60 series, the bridge of the TOS Enterprise (in the days when building a quarter section of it was budget restraining!), and even that familiar high-pitched whine of a 23rd Century transporter.
However, for some reason, Scotty is convinced that James T Kirk has hauled the Enterprise out of mothballs to come looking for him. Geordi and Riker exchange rather uncomfortable glances at this point because as they know, Kirk's been dead since the maiden voyage of the USS Enterprise-B, and Captain Montgomery Scott was right there. That transporter beam must have not only degraded so much that it lost Franklin but also took some bits from Scotty's memory as well.
The reality is that when Relics was written in 1992, Generations was still two years away and thus the story that would lead to the apparent death of Captain Kirk had not even been sparked let alone committed to a page. Scotty and Chekov might well have been editorial copy and paste to replace Spock and McCoy but without this convenient oversight of canon, Generations falls apart within 20 minutes.