Continuity mistake: When Khan contacts the Enterprise after Reliant's opening attack, Kirk has his uniform jacket unbuttoned to about mid-chest level (a little above his stomach). When Khan demands all information on Project Genesis be turned over, and Kirk responds with 'Genesis? What's that?' his jacket is now unbuttoned all the way down to his belt.
Continuity mistake: After the reliant explodes and the Enterprise is safe, Kirk goes to engineering to see Spock in the radiation filled chamber. During this entire scene, Kirk's tunic is open as Scotty and McCoy keep him from entering the chamber. As Kirk approaches the chamber to talk to Spock, the tunic is partially closed. The next shot shows the tunic all the way open as Spock walks towards Kirk. (01:39:55)
Continuity mistake: When Captain Terrell kneels down and says "What the hell is that?" Chekhov's gloved hand can be seen on the left side, but in the next shot Chekhov is already in the next room.
Continuity mistake: When the Enterprise is making a run for the Mutara Nebula, they show a scene in a photon torpedo bay. On the wall the sign says "TORP BAY 4." The problem is that the Enterprise only has 2 photon torpedo bays. Bay 1 (Starboard side) and Bay 2 (Port side). This version of the Enterprise has no aft photon torpedo bays.
Continuity mistake: When Admiral Kirk and party arrive in the transport pod, they dock on the port side docking port in the secondary hull. But the airlock they emerge from is in the docking port for the port side torpedo bay, as signified by the sign in the background saying "TORP BAY 2." You can see the docking port they are supposed to have docked at to emerge from on the side of the torpedo launchers, just above of the secondary hull. The reason for this is because they re-used the stock footage from ST:TMP when Kirk and Scotty docked at the same docking port.
Chosen answer: According to the novelisation of the film the Genesis project was initially designed to be capable of creating an entire solar system. While the focus of the project eventually narrowed down to altering an individual planet, the sub-routines necessary to create a star were still in place and were activated when the device detonated within the nebula.
Tailkinker ★