One of the signs that a story has jumped the shark is when characters who previously couldn’t do so suddenly gain the ability to fly. A story has really jumped the shark when a character suddenly acquires a flying motorbike. (I have no idea why this idea is so popular.) A story has really really jumped the shark when it’s an invisible flying motorbike.
I’m not talking about characters like Superman or Iron Man, they could always fly. I’m not talking about characters like Hagrid who has a flying motorbike at the beginning of the Harry Potter series.
I’m talking about characters who suddenly acquire abilities in order to invigorate a tired plot. This is never a good idea, if nothing else it frequently invalidates previous events.
The point is illustrated in Blood, Sweat and Pixels by Jason Schreier, a book that partly describes the troubled development of the computer game Star Wars 1313. One of the biggest stumbling blocks was the introduction of the character Bobba Fett half way through development. Bobba Fett had a jetpack, which meant that the levels had to be completely redesigned. Objects that were previously out of reach could now be flown up to, for instance.
SF is all about establishing a set of parameters and then exploring them. You can push your world to its limits (in fact, that’s the point of SF) but you can’t change the rules.
Once you’ve built your world you have to stick with it. If you’ve exhausted it, move on.
The First Number Not Nameable in Under Ten Words 