Could be a single game collapse or a season collapse, or a more abstract collapse like going from a dynasty to irrelevant for a long period of time suddenly.

As a Broncos fan the worst I've seen were the 08 and 09 seasons. 08 stings a bit more because it was likely the last straw with Shanahan in Denver. The roster was loaded and in the coming years Jay Cutler and Brandon Marshall would both be traded along with Tony Scheffler. Guys like Eddie Royal would become misused and be more special teams focused than as a deep threat. Just a total shit show.

09 sucked because we assumed after 6-0 playoffs were all but guaranteed, but going 2-8 over the last part of the year is just a brutal way for the season to end after so much optimism.

So what is the worst for your team?

Edit: Thank you for all the responses! I also wanted to throw in 06 for the Broncos. That one hurt majorly especially bc Jake was benched a little over halfway through the season for Jay Cutler who threw a game losing pick to the 49ers in week 17 to miss the playoffs. If Jake starts the whole season the most likely make the playoffs