"Disallowed Key Characters" error from bad cookies

[eluser]Unknown[/eluser]
The CI site I manage is accessed through a proxy server. One of the other sites accessed through the proxy sets a cookie that causes CI to choke and throw the "Disallowed Key Characters" error. I've looked at the code, and it's pretty strict about what it permits, and I was wondering what problems might result from loosening the restrictions a little bit?