If you click on a lookup field (for any entity), you can click on the magnifying glass or on the cross. You click quite easily on the cross, so that the existing value is immediately removed. This is because the width in the steps of a Business Process Flow is very narrow.

You cannot restore this with a button. You can only leave the record immediately and not save the changes. If it is a required field, you are in luck, because then the record will not be saved if the field is not filled. If it concerns a non-mandatory field, the record may now have been automatically saved and you can only look up the old value from the control history. That is very difficult for users and consumes a lot of time to recover the original value of the field. 

As a user you cannot see the content of the value without hovering over it. Also the cross and the magnifying glass are quite near each other. The wish of one of our customers is to make the content of the value more clear, or change the position of the cross so that it is not so near to the magnifying glass anymore. However, best option would be a redo button during the session that you are editing a record.

Thanks in advance.