If you imply that something will be a continuing problem unless dealt with, you cannot proceed to ignore it and move on.
The most common example of this rule being broken (which you will see all over the place) is when a character expresses that he/she really badly needs the bathroom, usually done as a cheap joke. The story then moves on and you spend the rest of the movie/episode empathetically feeling their pain with no resolution.
In City Of Ember when Lina and Doon discover the boat deploying mechanism designed to escape Ember, they activate it only to discover that the water levels aren't high enough and the first boat is obliterated. They then fail to shut off the mechanism and move on as if the problem will resolve itself. As a result the viewer can't focus anymore because they are under the impression that every passing second another boat is being needlessly destroyed.
See, in normal circumstances this sort of tension would be used to ramp up the stakes in any given sequence or at the very least get a laugh. But all too often writers neglect that if your viewer is along for the ride, you can't have your side mirrors fall off and not acknowledge it.
Did that metaphor make any sense? Eh, you know what I meant.