The Quebec police force is denying that they had undercover cops posing as protestors at the recent North American Security and Prosperity Partnership (SPP) meeting in Montbello, Quebec. Decide for yourself. The most interesting part is between 1:50 and 2:50. You can see the 3 masked protesters all of a sudden get nervous and allow themselves to get arrested. A CBC article goes into a lot more detail.
As if that wasn't enough evidence, check out this high-res photo showing the protesters wearing the same boots as the police officers.
What are the odds of the two protesters having the same boots? Secondly what are the odds of them also have the same boots as a police officer? The FAR more likely explanation is that they are all police officers! (Another one of the pictures in that gallery shows a third protester wearing the same boots).
Update: The cops caved.