PostNuke Forum (registration required).
You can easily enough enter and read on the forum, but if you attempt to post, preview a post, or edit your prior posts -> you cannot get past the AJAX - JSON error message.
Works in FF 1.5.0.3, Safari Version 2.0.3 (417.9.3), Netscape and even (sigh) IE 5.2.
Does not work in OW 5.1.3.