Here's another error I got today while using Visual Studio 2008
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Microsoft Visual Studio
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Error connecting to undo manager of source file 'C:\dev\Work\ReportBrowser\ReportBrowserWeb\SiteLayout.Master.designer.cs'.
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OK
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Found a good solution here
http://www.hanselman.com/blog/CommentView.aspx?guid=8bc798ae-da26-4864-b4e3-b68b081b4be1
1) Right-click the designer file and select delete
2) Right-click the aspx file and select Convert to Web Application