Plot hole: When Picard is talking to Guinan inside the Nexus she tells him that he can leave and go anywhere anytime and Picard and Kirk go back to Veridian III just before the launch of the missile. Now why didn't Picard go back to when they first located Soran on the Armagosa observatory and detain him there? Or if he wanted to he could have gone back even further to Earth and saved his brother and nephew.
Other mistake: When Worf tells Riker the Romulans were looking for Trilithium on the station, Riker acts as if he never heard of it before, and Worf explains that it is an experimental compound that the Romulans were working on. However, in the Next Generation episode, "Starship Mine", terrorists steal Trilithium resin from the warp engines to sell as a weapon, and Picard seems to know a great deal about it. (00:34:10)
Continuity mistake: It's established in TNG that Geordi's VISOR can see the entire EM spectrum, which several episodes showed to be a chaotic, indecipherable, visual frenzy that Geordi admitted he had to train himself to be able to comprehend. In 'Generations', however, the hijacked feed from Geordi's VISOR is shown as perfectly normal vision that looks like what you'd see from any standard camera.
Audio problem: In Stellar Cartography, Data has to reposition his chair to work on another panel. To do so without getting up he places his hands on the panel to pull himself across to the other position. As he does, there is a sound of an electric motor moving him. But as the scene draws to a close and Data gets up, you can see the chair is a dressed up standard office desk chair, with casters, no motor, and no tracks for it to slide on.
Other mistake: Historically, in the Star Trek universe, the doors to any room, including turbo lifts, opened automatically at a person's approach. The only time a turbo lift door didn't open was if there was no car there. This time Harriman touched a control at the right side of the turbo lift doors.