When something that was hyped to be great turns out to be mediocre, especially when you really should have known better in the first place.
This differs from jump the shark
because it refers to something that was never good to begin with, versus something that was good and went bad.
Snakes on a Plane
the movie is the example that inspired this phrase.
Jump the snake:
You can't say that Josh's blog jumped the snake, because it was so darn good until the last two posts. Then he got preachy, and the magic died. I weep for my loss.