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.