Sometimes it's a big event — a wedding, a birth, the discovery of a lost sibling. And sometimes it's just a desperate joke, a preposterous plot twist, or a hairstyle (cough — Sanjaya — cough). There are countless ways a show can jump the shark and turn undeniably, irretrievably, tragically bad. And when good shows go bad, there's usually no stopping their decline.

Here's the story of 14 shows and that dark moment when I realized they were heading into a tailspin.

-- By Matthew Gilbert, Globe Staff