It used to happen at TFC games in the supporters section along with the streamers. Streamers were eventually banned and I assume flares were too. One of my proudest moments (lol) was when I nailed a DC player in the head with a streamer and got props from the people around me. The guy didn't flinch of course because it was like getting hit in the head by a feather. I was just following the tradition of showering the corner with streamers. Mine just happened to hit the guy.
But I've also seen Powerade bottles and beer cans thrown at Freddie Ljunberg and Becks. And also at Gerry and Craig doing commentary when they set up in the stands behind the goal at the Dome vs. Galaxy and Becks.
This kind of thing had to stop, even for the streamers, if it's going to be large crowds. Because if streamers are allowed then punks and drunkards start to think they can throw any object. It could happen regardless, but can definitely be more frequent if streamers are allowed.
The flares thing can lead to who knows what. I was at a Juve vs. Fiorentina friendly at the Dome and dumbasses were setting off fireworks at the end of the game while Juve players were walking through the tunnel. It actually startled some of them. Of course, they chose the end of the game so they could just book it out of the stadium without consequences.
I and my coworker, whose father has season tickets, have traded stories about some of the absolute scumbags we've encountered in the ultras sections at BMO. These are the kind of guys that will think they have licence to do whatever the f*ck they want. Those are the guys that will do dumb shit on purpose and then you have immature guys who will think they're doing something funny but then it turns out to be reckless. The fire in Ottawa could be from either of those types.
Anyway, that Juve Fiorentina game I went to was memorable. My cousin caught a ball that was a wayward strike from Amauri. And as is tradition in Italy after some big moments after the game, they started to pretend they were Chippendales, including Del Piero: