It's not at the top of my list, but one which stung for me was the double knock in
April 2010.
We'd won 4-0 the week before, Chelsea had won 7-1. We were 1 & 2 on the table, with United 1 point ahead of Chelsea. Even with Arsenal trailing closely, it seemed to be all about United & Chelsea for the title. The two teams meet, and Chelsea win the game 2-1 on an offside goal by Drogba. I was fuming. A few days later we lead 3-1 v Bayern before Rafael gets a second yellow, and Bayern score the goal they need to go through. We draw v Blackburn the following weekend, then United won the rest of the games, but ended up losing by a single point to Chelsea, who had beaten us by an offside goal.
As for the CL, we had won it in 08, gotten to the final in 09, and this time around Inter beat Barcelona in the semi finals. We would've faced Lyon. Inter were brilliant that year, but we were also a brilliant side at the time. I feel like those two games potentially kept us away from a league title and another CL final.