Man United fans can hardly moan, if they deserved to win they would have scored a goal, and referees will always make mistakes because of the pace that football is played at, decisions need to be made in split-seconds, and if they don't see something properly then they can't be 100% sure of what decision to make they will obviously make some wrong decisions, fans just don't accept that at all. Slow-mo replays and replays from various angles will obviously make it look even worse for referees, it's just fuel for excuses for managers and fans.
Great day for the FA Cup, it is great that a team outside the top 4 and maybe even outside the Premiership will win the cup, will make it much more exciting and it's good for the game.