Also, to commuters on mountain bikes. Unless you've cut your bars down to a reasonable width stop filtering to the front.
Everyday I see some commuter who seems to have never filtered to the front try this and get stuck, often in an awkward spot between a bus and a lorry and I just feel bad for them. It's also annoying when I'm behind them and trying to get passed them while working(It doesn't really bother me but it is mildly annoying when I'm trying to knock off a red hot) It's just not safe to be filtering up to the front if you're that likely to get stuck half way there. Then you gotta slowly move backwards and try to fit yourself in somewhere safely and then traffic starts moving and etc... So I would certainly suggest trying to have a good awareness of the width of your bars. I don't run the hipster/courier style mountain bike bars cut down to nothing, I run regular road drops and they're narrow enough to get through almost all gaps in traffic, it's pretty rare that I can keep filtering up. But most mountain bike bars are fairly wide and will impede filtering. Just be aware of this and don't get caught in a tricky situation.

Also, if you're not filtering forwards right next to the sidewalk many pedestrians will not notice you and if traffic is at a standstill many will try to cross in between the cars. If you're filtering up quickly keep an eye out for these. If theres a bus between you and the sidewalk, which obviously impedes your view of said jaywalkers, be extra careful. I nearly clocked a kid head on when I was filtering up moving quickly the other day. Even though the law is technically on my side in this case(sort of, Im supposed to move up on the left rather than on the right or in between cars) I doubt running over a 14 year old will endear me to the authorities. Kid wasn't looking at all and had headphones on so he could hear me yell. I came to a screeching halt inches from him and he didn't notice anything at all. As a kid I was reckless, never flat out stupid like that though.