Audio problem: When Kirk says, "Stop energizers," watch his lips closely - he really says, "Stop engines." Mike Okuda points this out in the text commentary on the Director's Edition DVD. The reason the line (and also the one following it) was redubbed was because someone pointed out during post-production that the engines couldn't simply be stopped on a dime. (00:39:08)
Audio problem: When Kirk makes his "old wounds" remark to McCoy, just before he steps out of the turbolift, we can hear the sound used to indicate the turbolift is moving, but the door is already open.
Audio problem: On Ceti Alpha V after Chekov urges Captain Terell, "We need to get out of here," we hear Terrell respond with "But the tricorder..." But his lips actually say "But the child..." The line referred to a deleted scene so it was recorded over in post.
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 ★