A truly believable fictional universe is one that abides by the rules of its own creators. And the surest way to ruin a TV show's continuity is to suddenly change those rules—or ignore them entirely. What science fiction or fantasy show is the worst offender in this regard?
The most egregious example for me is Star Trek's Prime Directive. A starship captain's most solemn oath is that they will give their life, even their entire crew, rather than interfere with the development of an alien civilization. But with the help of some creative workarounds, James Kirk often found a way. ("This society is stagnant! It doesn't count!")
What about you? Are there other examples of shows that have frustrated you by blatantly violating their own rules? Please include pictures, video and links!