Let's talk about those teams that had it, the perfect storm of talent, momentum, and opportunity .... then just completely fucked it up. This could be due to a variety of reasons like bad coaching, poor management decisions, or individual mistakes. I'll start with some obvious ones to get the thread going
2016 Atlanta Falcons
28 to 3 blown lead in the Super Bowl need I say more
They went 10-6 and made the playoffs the next season but loss to Philly (Champs) in the divisional.
2016 Golden State Warriors
73-9 regular season, the Warriors blew a 3-1 lead in the NBA Finals
They bounced back to win back to back chips, but they came with its own conservatory
2016 Atlanta Falcons
28 to 3 blown lead in the Super Bowl need I say more
They went 10-6 and made the playoffs the next season but loss to Philly (Champs) in the divisional.
2016 Golden State Warriors

73-9 regular season, the Warriors blew a 3-1 lead in the NBA Finals
They bounced back to win back to back chips, but they came with its own conservatory