This sounds like you are adding a control to a page dynamically but not adding it again when the postback occurs. As the error message suggests, you need to reconstruct the page in such a way that the control structure matches when the page posts back.
I've seen this problem when someone attempts to have a button click handler add a control. When the page posts back, that button click handler doesn't execute and thus the control doesn't get added back into the control hierarchy. This results in a out-of-sync control tree.
Can you post just the code that relates to the control that is causing the problem?