That one episode in any visual media that even the most loyal fans find disappointment and contempt for.
Some TV series are great. Some aren't so great. Some web videos are great. Some aren't so great. What really determines how good a series is (bar Screwed by the Network syndrome), is how many Big Dud Episodes it has. A Big Dud Episode is that one episode where even the most loyal fans can't see past the disappointment and lapse in quality, even with the highest grade rose-colored glasses.