I should perhaps have put this in 'commuting' or 'advocacy,' but I reckon most of us here are at least quasi-rational, soooo --
there I am, this a.m., commute to work along our really quite nice river-side MUP. Need to know: at a given point, the path goes up what we call the 'hill from hell' -- at the top, must stop to cross a quite busy street which itself rises steeply, quickly, and with a blind curve from a bridge over the river -- path continues. On the flat preceeding the 'h from h' I pass a full-monty commuter (female, two panniers w/markers, one of those flourescent crossing guard vest thingies, helmet mirror, etc.). She's riding a moderate pace, etc. Up the 'h from h' I go, stop and wait for traffic to clear; up the other side comes a lady our age, obviously just beginning to bike; she stops -- a bit wobbly but careful -- traffic clears for a moment, and she sets out across the street. Meanwhile, Ms. has cranked up behind me -- I'm waiting for Lady to get across, AND I can hear oncoming traffic on the bridge; Ms. Passive/Aggressive simply goes right past me, and in doing so veers into path of Lady, who spooks, and does a slow half-fall; Ms. P/A blithely carries on; car coming up the hill off the bridge has to brake hard to avoid Lady, who by now is of course frightened/bewildered. Anyway, she's fine/everyone in Ms. P/A's wake recovers, and goes on their way.
Point: could have been a VERY serious accident here, it was that close.
Off I go, very quickly behind Ms. P/A again, waiting for clear-ahead to pass; approaching down another small hill is a city grounds-maintenance truck pulling equip. trailer, very slowly, keeping well right, following is another rider. Ms. P/A doesn't slow, but instead stays her lane toward the centre line and carries on, forcing the vehicle to slow, veer right, off its lane, and onto the verge - bewildering the following rider, who then sees Ms. P/A and just shakes his head.
Inevitably, of course, I'm shortly after passing Ms. P/A, and by this point think I have to say something, so pull up beside on a clear stretch and as politely as I can quickly describe what happened on 'h from h' and suggest perhaps a little more 'due care/attention' is in order! Response is a silent look that would do Medusa proud, so I shrug and carry on.
Gheesh! Anyway, I guess my point overall is that I could eat my arms off whenever I witness these kinds of things; we can and do all make mistakes/errors in judgement, but a little bit of basic consideration for others/adjusting our riding to the environment we happen to be in goes a long way.
That's it; sorry for the long post.