Custom server controls that use control state must call the Page.RegisterRequiresControlState(Control) on each request during before the state is persisted. Registration for control state is not carried over from request to request during a postback. Use the Page.UnregisterRequiresControlState(Control) method to ensure that control state is not persisted for controls that no longer need to be persisted for postback.

Internally, the Page.UnregisterRequiresControlState(Control) method is invoked when using the Control.RemovedControl(Control) method to remove a control from a controls collection.