With the clock ticking into the final minute and the scores level at 0-0, Mendes tried an audacious lob from 50 yards with United keeper Roy Carroll dashing back to his goal having raced out to clear seconds earlier.
Carroll made it back but inexplicably dropped the ball and it landed clearly a yard over his own goalline.
However, linesman Rob Lewis did not flag for a goal and referee Mark Clattenburg waved play on.
Afterwards, both our Head Coach Martin Jol and United boss Sir Alex Ferguson called for the introduction of video technology to determine such decisions in the future.
Martin said: "I don't want to be too harsh because everyone makes mistakes but we were 80 maybe yards away and could see it was in.
"Perhaps the linesman could have been nearer, but what can I do about it? Everyone is talking about technology and maybe that's a good thing.
"The referee is already wearing an earpiece - so why can't we just stop the game and get the decision right?
"It's the Premiership, a lot of money is involved and you miss out on two points.
"It is difficult for us because we can't say the decision will cost us this or that. But can you imagine something like that happening in a match between Arsenal and Chelsea? It would be a disaster."
Martin went on to explain that he had a similar incident go against him last season as boss of RKC Waalwijk.
"It was a game against Twente Enschede, the keeper missed the ball and just before the line or on the line he had the ball again, but they gave a goal.
"That was maybe two centimetres but this isn't even a yard, it's a metre."
Even Ferguson admitted the goal should have stood. "It's time to bring it (the technology) in - even if we only start it for goalline incidents. That would be the start of a new era, and I believe it is worth trying."
Technology was all the talk at Old Trafford after the failure of officials to award Pedro Mendes' 'goal' against Manchester United on Tuesday night.