Well I figured out exactly what causes this after getting someone that had the errors to install the firefox livehttpheaders plugin. It's the cookie size. Cookie too large, nginx returns a 400 and doesn't log it. Unfortunately there isn't much we can do about the cookie size. With the fb connect iframe stuff there are other parties setting cookies on our domain.

I tried in the past upping some buffers because I thought this might possibly be an issue, but it didn't help. Maybe I set the wrong ones. What specific setting can I use to allow large cookie headers?

Below is a session of one of the requests causing the error:

GET /forums? facebook = 1 &token = 56be59ba413134e28e2976e35545960c &user_id = 8 &app_name = mogul &fb_sig_in_iframe = 1 &fb_sig_locale = en_US &fb_sig_in_new_facebook = 1 &fb_sig_time = 1244858147.2827 &fb_sig_added = 1 &fb_sig_profile_update_time = 1232413305 &fb_sig_expires = 0 &fb_sig_user = 542007204 &fb_sig_session_key =deec04765ed50ee023ae5cf1-542007204&fb_sig_ext_perms=offline_access %2Cemail %2Cauto_publish_recent_activity &fb_sig_api_key = cf73a4d662d5ce97b44ec8a9f3121c9d &fb_sig_app_id=15420082636&fb_sig=b88fd355c67517fb5d67230415750609 HTTP/1.1 Host: facebook.offbeat-creations.com User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv: 1.9.0.11) Gecko/2009060215 Firefox/3.0.11 Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/ *;q=0.8 Accept-Language: en-us Accept-Encoding: gzip,deflate Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7 Keep-Alive: 300 Connection: keep-alive Referer: http://apps.facebook.com/beatycoon/wall/forums Cookie: __utma =131917208.1556222345328091100.1244852730.1244856612.1244858119.3; __utmz=131917208.1244858119.3.3.utmcsr=apps.facebook.com| utmccn=(referral)|utmcmd=referral|utmcct=/beatycoon/; user_542007204= %5B%7B%22activities%22%3A%22%22%2C%22affiliations%22%3A%5B%7B%22nid %22%3A67108883%2C%22name%22%3A%22Seattle%2C%20WA%22%2C%22type%22%3A %22region%22%2C%22status%22%3A%22%22%2C%22year%22%3A0%7D%5D%2C %22birthday%22%3A%22December%2024%22%2C%22books%22%3A%22%22%2C %22current_location%22%3A%7B%22zip%22%3A%22%22%7D%2C %22education_history%22%3A%7B%7D%2C%22first_name%22%3A%22Chia%20Chin %22%2C%22interests%22%3A%22%22%2C%22meeting_for%22%3A%7B%7D%2C %22meeting_sex%22%3A%7B%7D%2C%22movies%22%3A%22%22%2C%22music%22%3A %22%22%2C%22name%22%3A%22Chia%20Chin%20Lee%22%2C%22pic%22%3A%22http %3A//profile.ak.facebook.com/profile5/772/114/s542007204_148.jpg %22%2C%22pic_small%22%3A%22http%3A//profile.ak.facebook.com/ profile5/772/114/t542007204_148.jpg%22%2C%22relationship_status%22%3A %22Married%22%2! C%22religion%22%3A%22%22%2C%22sex%22%3A%22male%22%2C%22status%22%3A %7B%22message%22%3A%22%22%2C%22time%22%3A0%2C%22status_id%22%3A0%7D %2C%22tv%22%3A%22%22%2C%22uid%22%3A542007204%2C%22wall_count %22%3A305%2C%22work_history%22%3A%7B%7D%2C%22pic_square%22%3A%22http %3A//profile.ak.facebook.com/profile5/772/114/q542007204_148.jpg %22%7D%5D; gtl=131174; photos_542007204=%5B%7B%22subject%22%3A %22565010917%22%7D%2C%7B%22subject%22%3A%22603403123%22%7D%2C%7B %22subject%22%3A%22604053658%22%7D%2C%7B%22subject%22%3A %22636124705%22%7D%2C%7B%22subject%22%3A%22639286473%22%7D%2C%7B %22subject%22%3A%22644531360%22%7D%2C%7B%22subject%22%3A %22679766796%22%7D%2C%7B%22subject%22%3A%22700195562%22%7D%2C%7B %22subject%22%3A%22708288958%22%7D%2C%7B%22subject%22%3A %22708288958%22%7D%2C%7B%22subject%22%3A%22746248293%22%7D%2C%7B %22subject%22%3A%22746248293%22%7D%2C%7B%22subject%22%3A %22757564008%22%7D%2C%7B%22subject%22%3A%221165467950%22%7D%2C%7B %22subject%22%3A%221165467950%22%7D%2C%7B%22subject%22%! 3A%221165467950%22%7D%2C%7B%22subject%22%3A%221182339204%22%7D! %2C%7B%2 2subject%22%3A%221223236240%22%7D%2C%7B%22subject%22%3A %221223236240%22%7D%2C%7B%22subject%22%3A%221223236240%22%7D%2C%7B %22subject%22%3A%221303717901%22%7D%2C%7B%22subject%22%3A %221303717901%22%7D%2C%7B%22subject%22%3A%221313378551%22%7D%2C%7B %22subject%22%3A%221313378551%22%7D%2C%7B%22subject%22%3A %221347486368%22%7D%2C%7B%22subject%22%3A%221347486368%22%7D%2C%7B %22subject%22%3A%221347486368%22%7D%2C%7B%22subject%22%3A %221444025780%22%7D%2C%7B%22subject%22%3A%221507201538%22%7D%2C%7B %22subject%22%3A%221536746061%22%7D%2C%7B%22subject%22%3A %221536746061%22%7D%2C%7B%22subject%22%3A%221536746061%22%7D%5D; __utmb=131917208.4.7.1244858119139; __utmc=131917208; mogul_game_session=BAh7DTogX3JmYWNlYm9va19mYnNlc3Npb25faG9sZGVydToiUkZhY2Vib29rOjpGYWNlYm9va1dlYlNlc3Npb24CFAEECHsPIhhAbGFzdF9lcnJvcl9tZXNzYWdlMCILQHF1aWV0RiIQQGFwaV9zZWNyZXQiJWRjM2E5OTAxZDVkNTYxOWRjOGMzYzllMGE5OTcxMjkyIg1AbmV0d29yayINZmFjZWJvb2siEUBzZXNzaW9uX2tleSInZGVlYzA0NzY1ZWQ1MGVlMDIzYWU1Y2YxLTU0MjAwNzIwNCIVQHNlc3Np! 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 %3D%3D--c75e1ae9b191ce2f330ce702f2549ece203baff8; fbsetting_cf73a4d662d5ce97b44ec8a9f3121c9d=%7B%22connectState %22%3A1%2C%22oneLineStorySetting%22%3A1%2C%22shortStorySetting %22%3A1%2C%22inFacebook%2! 2%3Atrue%7D; cf73a4d662d5ce97b44ec8a9f3121c9d=dca5f61a4d674f57! b6ce07e3 e1261045; cf73a4d662d5ce97b44ec8a9f3121c9d_user=542007204; cf73a4d662d5ce97b44ec8a9f3121c9d_ss =d867120f5c1cc5eac268e3cdae6c43cc; cf73a4d662d5ce97b44ec8a9f3121c9d_session_key =deec04765ed50ee023ae5cf1-542007204; cf73a4d662d5ce97b44ec8a9f3121c9d_expires=0

HTTP/1.x 400 Bad Request Server: nginx/0.6.34 Date: Sat, 13 Jun 2009 01:55:53 GMT Content-Type: text/html Content-Length: 173 Connection: close

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,2757,2891#msg-2891