This message crops up occasionally referring to a designer.cs file. It is tricky to replicate this issue as it happens at random. However it typically appears when you are editing an aspx/ascx file and hit F5 to debug the project. It occurs in Visual Studio 2005 SP1, VS 2008 RTM as well as SP1.
Fix:
Found a fix to this issue from here
Steps:
1. Right-click the designer file and select delete
2. Right-click the aspx file and select “Convert to Web Application”
– Mehul Kothari
issuesandfixes 