Speaking of sidewalks, groups of people walking together and taking up more than their half of a sidewalk or waiting until the last second until moving over.
I'm usually pretty quick to fall behind my wife for a second to allow others coming the other way room. I'll always do it when the sidewalk is only wide enough for two, but also almost always when it's wide enough for three and there's a couple coming towards us - someone has to move, and I don't mind being the one in that case. But I'm not moving so a group of three+ can stay walking side-by-side the whole time. My wife and I are both taller than average and could each stand to lose a few pounds, so we've decided that we're not going to lose games of chicken to these kinds of people - if they want to actually run into us, have at it.
Just last week, we were walking together on a sidewalk wide enough for four (with snow on either side of the walk) when a group of four twenty-somethings approached. Apparently, none wanted to be one of the ones that drifted back for a moment, because they weren't showing any signs of splitting up their 4-wide. I made eye contact with the one right in front of me and literally asked, "What's your plan here?" just before they moved behind their friends at the last second. Did they seriously expect us both to walk in the snow?