Internal Consistency– the unbreakable rule in science fiction and fantasy

I like to find my way around rules and ideas in storytelling and I enjoy it when others do so as well. Often what we call rules are little more than ideas that usually work, but there are a set of rules that you must be very aware of and never break. This are the internal rules of the story that you set up.
This is of course extra important when you are dealing with science fiction and fantasy that have internal rules that often break the rules of the world as we know it, but they aren’t the only one. You can’t write a romance set in the 18th century and then have one of the characters call for help on a cell phone.
An example of a movie that fails to do this as well as it should is “Next”. The main character can see two minutes into the future, except when it is directly connected to a specific woman then he can see farther. As the movie goes on though this rule are more and more ignored. The writers were aware of it of course and attempted to write around it but at least for me they weren’t successful.
Perhaps the most frustrating of internal consistency being tossed out the window is in the end of movies where the heroes have been trying unsuccessfully to stop a plot then at the end something often not as interesting as the first works because it is time for the movie to be over. You knew how long the movie needed to be when you started, why not set up the ending?
For science fiction we run into the collective amnesia syndrome when it comes to technology. Some technology, superpower, or other element is on the show. Perhaps a time machine, or a character who can heal people with her blood, but everyone seems to conveniently forget that when the time comes. They have the time show episode and then no one bothers suggesting that perhaps the solution to the problem this week might be using the time machine from last week, and while I understand you can’t expect the writers to always see this, and you wouldn’t want them to fall back on the same technology to solve every solution why not simply explain why it doesn’t work rather than simply pretending it doesn’t exist?
In the end most of the time people will accept any stretch of logic so long as it is written into the basic premise of the show, but nothing will turn of readers and viewers of a show faster than the writers ignoring the rules that they set up, so while the rules of writing can be ignored, the rules of your writing can’t.