Well this morning, I happened to see that it had arbitrarily banned someone on my flist, too (secritcrush -- you probably didn't realize you were banned but I fixed it now). What you do to fix it is to hover your cursor over a friend's little person icon without clicking anything. In a couple of seconds, a menu will pop up with the option "unban user." If the user isn't banned, it'll say "ban user" instead.
ETA: Courtesy of frigg : There's also a "ban-unban" feature under "Friends", where you can click and see who are on your banned list. Any accidentally banned friends might show up there as well.