VMK007:

This may be a stupid question, but are you debugging when you are trying to do this?

 Of course not, everything is ok when the page in Source View. Even I have restarted VS2005 many times, but the problem still exist when the page in Design View, weired!

Mikhail Arkhipov (MSFT):

If you type in design view, does undo button become enabled?

Nope, both Undo and Redo buttons are disabled. And I just noticed that, if I make a copy of both *.aspx, *.vb and rename it, the copied page has no such problem. How could this happen?

This is a well known bug in VS2005, and has existed since before release. Maybe someday MS will fix it.

 http://forums.asp.net/thread/1384097.aspx

http://forums.asp.net/thread/1139188.aspx

 http://forums.asp.net/thread/1193304.aspx

It has something to do with the refactoring failing.

Here is a little history of the bug (it used to give an error message until they "fixed it" now it doesn't give a message, but the ID still won't change )

http://www.west-wind.com/weblog/posts/3192.aspx

 and the bug in MS:

http://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=101992

The best solution is to delete your website cache as outlined in the above link.