Will Pharell was in transit in the Paris airport and he walked in the McDonalds. It was 6.30 in the morning, the restaurant wasn’t open yet and at that time of the day, they only serve breakfast anyway. He wanted to order a Big Mac and didn’t get it, he made a song about it and the video ended on Youtube (over 1 million views in 3 weeks). McDonalds France tried to answer, but this wasn’t really a success.
UPDATE: a n ice overview of what happened can be found in a nice presentation