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It would help if you posted the code that triggers this error, but the common reason for this exception is that you are trying to access a variable outside of it's scope, for example if you declare a variable inside Page_Load() and then try to access it in another method in your class.
I have a live working EPiServer CMS 5 website.
After making some changes to the Globalascx.cs file on my development website I recompiled and copied the dlls for my website (not the episerver dlls) to live website. Then i got the error 'CS0103: The name xxx does not exist in the current context' when i browsed the live website. I have checked the ascx and master file on the live and dev servers and they are identical, also the 'xxx' does exist and did exist prior to my update.
Because it is the live site I have restored the original dlls and now the website works again (obviously without my intended changes).
Why would an area not changed by my update suddenly cause an error for something that already existed? Is there something else I should have updated in addition to the dlls?