This error occurs mostly when you are working with Dynamic controls, e.g. controls that you create at run time and add them to the Controls() array of your page (or form, or panel, etc). What this is saying is that you are either trying to remove controls from the control tree or add to them, and you cant do this as the viewstate doesnt match.
A good example of this is when you are working with datagrids where you edit the data inline (dg.edititemidex = e.item.itemindex) but you forget to call the dg.databind() method in the Method that calls the following code, if you try hitting the edit button, the error you are getting will be thrown.
If you are still having some problems post some source code and ill take a look at it for you.
"The one language all programmers understand is profanity."