I was working with a solution created in VS2008 targeting .NET 3.5.
The migration to VS2010 was OK, It compiles and runs OK. Then I did a lot of development without any problems.
Today I've changed one word on the aspx of a page I did not change before and broke my build. VS2010 complains that a lot of controls doesn't exists with the message: "The name '...' does not exist in the current context".
I did code this page referencing components directly because in VS2008 the Wizard is like MultiView, now It is like data-bound controls and one must use FindControl to access anything inside it.
I see two problems here:
1 - The control should not introduce breaking changes when targeting an old framework;
2 - The migration wizard did not warned about it;
I compared the source code with the previous working version and all control declarations were removed from designer.cs
The problem could be only with the form designer that not did not use a diferent behavior for the targeted framework version.