Error connecting to undo manager of source file "designer file here"

This error happens a lot with visual studio, up to and including VS2008. The reason being that VS keep track of the form designers in debug mode by creating 'designer' files for each ASP.NET or each Windows Form etc...

This usually occurs, for whatever reason, when the designer file cannot be updated, usually being its corrupted. To resolve this error, simply do the following...

ASP.NET

1) Right-click the designer file and select delete
2) Right-click the aspx file and select Convert to Web Application

Other Project Types

Exclude the file from the Project, recompile, then re-include, the recompiles.