Arsenal manager Arsene Wenger criticised referee Martin Hansson for allowing the controversial goal which left his side trailing in their last-16 Champions League clash with Porto.
Polish goalkeeper Lukasz Fabianski was caught out of position to gift Porto an early lead before Sol Campbell equalised.
However at the start of the second half, Fabianski picked up what was deemed a back-pass from Campbell, with Hansson - the Swede who missed Thierry Henry's infamous handball in France's World Cup play-off win over the Republic of Ireland - letting it be taken quickly as Falcao swept the ball into the net.
"It is difficult to swallow a defeat like that," said Wenger. "The goals we conceded were difficult to imagine in a Champions League game."
Wenger lamented: "What can you do about the second goal? The back-pass was accidental, whenever do you see the defender kick the ball back with his toe?
"The ball hit Sol, it was not on purpose and it has to be intentional to be a free-kick. It is difficult to understand how the referee can interpret that.
"Then, on an indirect free-kick, if you allow the team to play quickly, just five metres from the goal, how can you defend that? It is better than a penalty. It was unbelievable that he allowed Porto to play straightaway and push the ball into the net.
"I have never seen that and I have been in the game a long time. It is difficult to understand. It is completely inappropriate that he allows that in such a situation.
"When the referee gives the free-kick he has to allow us a chance to defend it, otherwise it is better to give a goal straight away.
"From a situation where there was no goal chance at all, and no free-kick as well, the referee gives them just a goal.
"It is difficult to understand, but maybe I am not intelligent enough."
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