After saving context, UITableViewCell's data is not been refreshed because binding is not triggered for UITextFields contained by cell. Have you seen any similar side effect so far?
The problem was that I executed really a lot of operation / query on persistent store from observeValueFromKeyPath. And observeValueFromKeyPath was triggered too many times. So in somehow the system in the background might just get overloaded, and I think this produced the strange side effects.