Revealing mistake: The scene of rioting in the UK features a load of police officers running through a crowd. All the officers shown are wearing black tunics and custodian helmets worn with chinstraps. This is obviously old stock footage from a protest in the 80's or early 90's as nowadays, (and especially in 2012) the tunic has not been worn as general wear for well over 15 years, and Brit cops are now better equipped for protests with body armour, NATO helmets, flame retardant overalls, etc.
Revealing mistake: If you look under the car when the meteors attack Jackson's car. you can see the green screen.
Revealing mistake: A wire is visible when the car flips over in Paris. The shadow is easier to see.
Revealing mistake: During the initial collapse sequence when John Cusack is driving the limousine against the tide of the collapse, the limousine ramps off a piece of floor that is collapsing inconsistently. The wheels stop spinning the moment they leave the road.
Answer: Because it's the 21st December in the MAYAN calendar. In the calendar most people use, it was actually July.