Alternative intros.

Sometimes a TV show has a special episode and they create a alternative intro for it - Star Trek Enterprise in a mirror Darkly part 1 and 2. & Friends season 6 the one that could have been part 1 and 2. Are 2 examples. (Feel free to list any other show/episode with an alternative intro.) But sometimes when these episodes are repeated on tv (& some othe places) the alternative intro is not shown and & regular one is use.