Can't say I have the same issue, I'm actually glad to encounter someone I haven't seen in a while, I'll even say hi to those I'm not good with, that's sort of guy I am.

I'll stop, shake hands, and have a chat with those I'm good though, just the other day while taking care of some stuff on university I'm transferring to I run into old colleague of mine, I haven't seen her in quite some time, two - three years to be more precise. We went to have a cup of coffee, pretty soon I'll be heading on the same university as her, so she explained me few things, gave me some useful tips.

As for worst encounters, there have been quite a few awkward ones, running into ex girlfriend for instance.