For me too many.
Reading in the newspaper that JV Cain had collapsed and died on the practice field.
Reading in the newspaper about Steve Little's car accident presumed to be a suicide attempt after being cut.
Watching Louis Lipps for the first time knowing the Cards took Clyde Duncan over him.
The '84 loss at RFK mentioned earlier, they just handed the game to Washington, 10 penalties on us, the only one they called on them erased a sure sack by Curtis Greer(they called a false start on a late whistel). Theismann got away with 3 blatant intentional groundings, the sack by Junior mentioned before was AFTER 2 of those uncalled groundings, he couldn't hear the whistle, and had his back to the ref who was waving his hands that the play was over, horrible call, although it wasn't on the game winning drive it was in the first half I think. The Cards lost that game when O'Donoghue had to run out and try to kick a ~50 yard FG and missed.
The Viking game in the playoffs, as great as it was to be there I've never felt more helpless as a fan, we couldn't stop their offense and we turned the ball over so many times in the first half it became painful to watch.
Being the 2nd NFL team in history to lose to Tampa Bay has to rank up there too.