Holy crap, I found what the mistakes were.
On one transition from one gallery image to another, it wasn't set to the default "Dissolve". It was somehow set to a name to a random scene, I don't know how that happened.
The other bug, there being no cg13 defined is bc there's no cg13 and I forgot to keep it from spawning a "next" button on cg12. This is why one mustn't code at 4AM. Thank you for the bugs listed out, immediately fixed them and will get a build in an hour!