Windows Vista - http://www.vistaarticles.com
While adding a list of proxy exceptions list is truncated
http://www.vistaarticles.com/articles/158/1/While-adding-a-list-of-proxy-exceptions-list-is-truncated
By 
Published on 11/23/2006
 
Describes an issue where a list of proxy exceptions in the Group Policy Object Editor is truncated if the list contains more than 255 characters.

While adding a list of proxy exceptions list is truncated

Because of a limitation in the way INS files are read, the proxy exception list is limited to 255 characters. The Do not use proxy server for local check box appends the text "<LOCAL>;" to the end of the list. This text is eight characters long, and the 255 characters must include these characters. Therefore, when you click to select the Do not use proxy server for local check box, the total length of the exception list is actually 247 characters.

Details: http://support.microsoft.com/kb/302224/en-us?spid=2073&sid=global