Given that a lot of television shows run for close to a decade, you have to remember that writers are dealing with thousands and thousands of pages of character arcs and plot threads. It's not exactly surprisingly then, when now and again, television writers forget about certain threads because they've become entangled with some more interesting ones.
And it's kind of understandable that this sort of thing happens - perhaps more so than when it happens in film - given that television shows are almost always written by a team of people (included the dreaded "guest writers"). There are so many people involved, pitching ideas and coming up with crazy new concepts, that sometimes ongoing plot threads can just get lost in the midst.
Usually fans of the show will direct TV writers to the fact that they've forgotten to resolve a plot thread or will let them know that we still don't know what's happened with Jake's cancer diagnosis. Then the writers will try their best to write it back in, like, what, we knew that all along, we were just tricking you. There's also the possibility that a show was cancelled before it had a chance to satisfy our curiosities (seriously, does Jake have cancer or not?!) and the writers didn't get the chance to explain.
Here's 10 TV plot threads that - for one reason or another - were never resolved.