Symptoms

"SmartObject GUID] could not be found" error in Designer and at run-time.
 

Diagnoses

This occurs when a SmartObject, that is referenced by a form/view control, is moved or deleted, either manually or by selecting 'Regenerate SmartObjects'. Besides at run-time, the error also occurs in Designer, which means that the offending control cannot be removed to prevent the exception, because when the exception gets thrown the rest of the page stops initializing.
 

Resolution

Selecting 'Regenerate forms...' or 'Generate forms' should update the offending controls. It is recommended to regenerate SmartObjects before this step as well, if they have not been already.