alika - 12:30 am on Dec 4, 2011 (gmt 0)
I more carefully removed the plugins one by one -- and found the culprit. It was Addthis that was adding the weird characters
Whereas before I focused more on plugins I thought were the culprit and skimmed those I thought was dependable, I now took the time to remove the plugins and went through several pages of the site.
I wasn't expecting it to be Addthis because I thought they were more or less updated more often. But I deactivated Addthis and the problem went away; activated it again and the weird characters in the URL happened again.