There aren't many pro disappointments bigger than 2012 that I can think of .... either my team never was in it or they didn't squander a lead like the Nats had. What didn't help was I was at Game 4 for Werth's HR. Boom ... down in flames.
I will go down a partial list.
64 Phillies collapse. Remember listening to the one game when Chico Ruiz stole home for the Reds with Frank Robinson batting. I was 6.
77 Phillies. Attended games 3 and 4 vs LA. Game 3 was the worst. Blown call at first base and then Luzinski botched a ball in LF. Had always been lifted in those situations during the regular season. The 77 and 78 teams were better than the team that won in 80.
1980 Stanley Cup finals. Watched in a bar in Margate, NJ. Bad OT ending with a missed offsides call.
Various Eagles playoff debacles.
Caps failures in recent years.