This is probably reproted, but its driving me mad.
I spend half an hour writing a detailed post, then when I it "Post new topic" I often get this:
Only members with sufficient permission can access this page.
A security error has occurred with authentication.
Please ensure that all cookies are enabled on your web browser, you are not using a saved or cached copy of the page, and your Firewall/Proxy settings are not hiding or masking your IP address.
The killer is if you hit the back button, your post is gone. So you have to retype the whole thing. It always works the second time.
I get this error at least once a day.