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:
<font color=red>
Error Error
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.
</font>
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.