Okay, here is one that gets me all the time, and which I have to look for the solution all the time.
To regenerate the designer page
1. Delete your bogus deisgner.cs page
2. Right click on the aspx file, and select Convert to Web Application
The solution is found in the link below, by John Q. Smith
http://social.msdn.microsoft.com/forums/en-US/csharpide/thread/128202d7-...