Jump the Shark
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The moment in a long-running serial canon when the source is generally felt to have dropped sharply in quality and never recovered. This is also the point when fanfiction usually begins generating alternate seasons or fix-its, or ignoring plot developments/calling actual plot developments non-canonical.
Of course, opinions will differ as to when and if this point is reached. If a fandom lasts long enough, you can end up with multiple communities of denial fans, each denying or ignoring different aspects of canon.