Blast Furnace Canada has an interesting post about a recent prank by the Belgian national broadcaster that went awry, and raises the question of what Canada would do if Quebec unilaterally declared independence. Would Ottawa be able to rely on the Quebec divisions of the Army? Would it matter - would a military response be possible/likely/necessary?
In this discussion, I'd simply point out that there's an immense amount of trouble that Quebec could cause as an independent state. The Quebec government has, on occasion, "accidentally" published maps showing the wrong borders with Newfoundland and Labrador. If you assume Quebec nationalism was in full flower (naturally, this would be necessary for a declaration of independence) all of the sudden a certain irredentism among Quebeckers is not out of the question. "Rescuing" the francophone parts of Ontario or New Brunswick would almost certainly be raised as a possibility.
So what's the nightmare scenario? Assume Quebec did leave, how bad could it get?