Are you pissed up with this kind of errors on websites like Paypal?
All those cookie tracking systems that are not efficiently managed or are too stringent in security violations run into this kind of errors all the time. Here is one example from Paypal.com
400 Bad Request Your browser sent a request that this server could not understand
Your browser sent a request that this server could not understand.
Size of a request header field exceeds server limit.Cookie: KHcl0EuY7AKSMgfvHl7J5E7hPtK=f6Siw-KN8p7lDN_hD52TJYDf......
What to do?
Clear your cookies and you are ready to roll.
Try to avoid working in multiple Tabs when accessing paypal in a logged in mode.
I hope that helped.