[_ck_]

The proper header for the page apparently is not being sent until there is a logged in user.

I’ll see if I can reproduce this. Is it in kakumei theme?

Update: this works fine on my setup:

http://bbshowcase.org/forums/topic/put-your-test-posts-here?bbtheme=kakumei#post-302

You might have something sending whitespace before all the headers can be sent so the page encoding gets broken until the user is logged in. Are there any errors visible when you do a “view source” or any javascript errors?