Just saying that you two aren't alone with this, but I haven't seen it here in a while. The last time I got that "ban" was maybe two months ago, just guessing at the timeline. The only way for me to duplicate a false ban like that is to go here:
http://www.4chan.org/bannedMeaning that if I click on the link after I post it, I see the ban page, but if I visit that URL by manually typing it in, it redirects me to the front page, signifying no ban.
That reminds me, a couple of months ago I had a possibly related error persist for several hours. I'd check the ban page and it started to load without actually displaying the frame where it gives you your ban reason. At the same time I was able to post, so to this day I don't know if I was actually banned for a day - from a board it just didn't occur to me to post in for testing purposes - or if this was another of them false positives, just without even a placeholder ban text that time.