The Header and Footer of your theme is a "Global Element". It shows on every page, and it can be updated while editing any page. For this reason, if two people are simultaneously making changes to the header on different pages, there will be a conflict.
If the Header or Footer has been changed and saved by a different user or on a different page, AND you have your own unsaved changes to the Header or Footer, upon saving you will be notified that while the changes to the page body will be saved, your Header/Footer changes will be lost.
The header or footer has been updated by email@example.com. Your changes to the header or footer have been lost.
Our system automatically checks every 2 minutes that the Header and Footer you are editing has not been updated and saved by another user. If the Header or Footer has been changed on a different page, or by a different user, you will be notified that you may no longer update the header or footer until you save your changes and receive the updates to the Header and Footer.