Since this is the preeminent post on WordPress Precondition Failed, I thought I would add this hint.
I was receiving Precondition Failed when trying to view WordPress blogs NOT using the Bad Behavior plugin.
If you use the iOpus iMacros browser plugin, it will add the user agent string (browser identification) “iOpus-I-M;” One place you can view what user agent information your browser is giving sites is at Anonymouse, click the “Calling Card Without Anonymouse” link.
The XP/IE6 registy location to check for and remove offending strings is:
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\User Agent\Post Platform]
After removing the iOpus-I-M DWORD key, I was able to view these WordPress sites again.
