Modals and User Experience

The question that I often come up with is: should we reset the state when users close the modal? For example, imagine there is a form for adding a new customer. The form can have a dozen inputs. Now, when you filled most of them but then accidentally close the modal, would you lose your data? That's obviously not a good experience. On the other hand, what if you want to add a new client without saving the previous one? So we need to meet both cases.