This is interesting.  The Star reports 3 protesters wearing bandannas and carrying weapons wandered into a section of Montebello protesters and were confronted by the protesters.

Someone shot a video which is up on YouTube. As the Canadian Cynic points out, there is a lot more credibility to these guys being undercover cops than many people discussing this are prepared to acknowledge. Watch the behavior closely, watch it with the sound on, and then with the sound off.
It is also interesting these three are dressed as Black Bloc, a generic international loosely affiliated (affinity) protesters, they’d blend into any demonstration.
I think The Star headline is a big disingenuous, undercover police are common in a protest so I don’t think they have to be accused of anything. “Police accused of using provocateurs at summit”

Of course police put their own people in a crowd. What is interesting is that when protesters confronted them, several things did and didn’t happen. 
Police in plainclothes in a crowd at a leaders summit would need some form of identification - the yellow triangle on the soles of the shoes makes sense.
If they are posing as protestors, a badge in their pocket, an earpiece, haircut, stance, many things can be a definite tell.
It only takes one person in a crowd to set things off. Why three? And why wander up to union protesters, there was enough action down the street, pepper spray, tear gas and rubber bullets. If they were looking for a fight, and were legitimate anarchists, they would know where to go.
These three are relatively fit. Their behavior and the behavior of the police line is off as they move toward police, pulling up the bandannas and saying something quietly to the armed officers. Legitimate protesters stand between them and the uniformed police line. 

It is interesting that some bloggers aren’t able to wrap their head around police tactics and need to dismiss this story as just another ‘moonbat’ one. It is interesting they assume everyone who attends protests are the bad guys and must be dismissed. It is interesting that to some people, disagreement of ideology means people attending a protest are not legitimate.
It’s good police work to put their own people in a crowd. Why they needed to look like they were escalating is anyone’s guess. Practise? Legitimacy? Discrediting?
The undercover officers were not supposed to get caught by peaceful demonstrators. Canadian Cynic:

JUST PLAIN CURIOUS: There are a few things about that video that I find more than a little odd. First, despite the fact that one of the provocateurs clearly has a rock in his hand, and numerous protestors are pointing this out in loud voices, the police that are only a few feet away do nothing about this. A masked (and alleged) protestor is wandering around holding a lethal object, and the police seem blissfully unconcerned. Weird, no? You’d think they’d, at the very least, be warning the “protestor” to immediately drop the rock but, strangely … nothing.

Shortly thereafter, the provocateurs end up next to the police line, and it’s clear from the video that the one is still holding the rock! Even as he’s standing immediately next to a police officer, he hasn’t been told to drop it. How retarded a law enforcement official do you have to be to allow an allegedly dangerous (and masked) protestor to walk up to you with a rock in his hand, and not ask him to put it down? But it doesn’t end there.

Immediately thereafter, the provocateurs quietly push their way through the police line, and are “arrested.” But on what possible charges? Until now, the police have been delightfully indifferent to masked protestors wandering around, carrying rocks. It hasn’t seemed to bother them so far. Now they decide to arrest them? For what conceivable reason?

And, finally, I find it curious that, as they’re being arrested, the police aren’t removing their bandannas. You can see clearly in the video that the tallest of the three, as he’s leaning against the wall and is then being taken down for the arrest, is allowed to remain masked. It’s hard to see what happens after that, and it’s annoying that the cameraman didn’t follow the procession out to the van to see if the bandannas came off anytime during the march. But one might reasonably ask whether it’s standard police procedure to not expose the people you’re in the process of arresting, given that it would have been such a trivial thing to do.

Update: Trying to chase down the yellow triangles on the bottom of the boots.  Police Supplies Canada sells boots with CSA white stickers.  (Canadian Standards Association) The stills taken of the three men being ‘arrested’ are linked in the comment section. 

The American standard compliance for safety boots is indicated with yellow triangle stickers.  ANSI Z41. (The American National Standards Institute)

Big City Lib notes the Vibram TM (an Italian company) also is also yellow.

The soles are cleared up, now where to tread?