Why Is The Client Cookie Autopopulating Text Box With Special Characters?

Ok, so - here's the issue. In my application, the Request Cookies Collection is returning the value of a session id which no longer exists but must have been stored in a client's cookie somewhere. Upon landing on the page, the parameter value of the request is being populated into the login.aspx text box for the user id - causing an error because the value being passed in is %23%23%23%23. In the trace info, the value is being stored in the cookie and in server variables. All of the code containing any information about user id in my application has been commented out - but the value still remains on the client's site, even after redeploying the commented out cookie code and additional code to clear those values from the login.aspx. Is there some kind of IIS setting or some kind of web config setting that is causing this value to be stored somewhere on the server? I shouldn't have to write any additional code to fix this issue, although I have implemented a workaround that redirects ...