Empty POST requests and IE7
I experienced a serious issue having to do with POST requests, and the only browser that seemed to be affected was IE7 RC.
After submitting form data as a POST request multiple times to the same page, my PHP test page would return an empty $_POST superglobal, meaning no data was received. Strangely, this only occurred if I submitted the form more than once within 15 seconds.
The first form submit always worked as expected, but any subsequent submit within the next 15 seconds came up empty. If I waited beyond the seemingly arbitrary 15-second limit and submitted the form, it again worked as expected.
Furthermore, I noticed that the issue only manifested itself on my machine, the test server. If I submitted a request using the same test script on the production server, IE7 worked flawlessly.
I compared settings in my php.ini, htaccess, and httpd.conf files, with no luck. I then decided to check the versions of everything, including PHP, Apache, and all installed modules and extensions. The test server had the latest versions of Apache, PHP, and most modules installed, while the production server was one or two minor revisions behind on each.
One of the loaded Apache modules I found with differing versions was mod_auth_sspi, which is conveniently hosted here. The test server used mod_auth_sspi 1.0.4, while the production server used 1.0.3. After downgrading to mod_auth_sspi 1.0.3, the annoying empty POST issue disappeared.
If you're running Apache and using mod_auth_sspi 1.0.4, I strongly suggest writing a quick script to submit data as a POST, and testing it with IE7. If you notice any strange activity at all, consider rolling back to 1.0.3. It may save you a serious headache later on.
It's possible that this is an issue with IE7 RC. If so, it may be fixed in the final version.