editingold - Warning: You are editing an out-of-date revision of this page. If you save it, any changes made since this revision will be lost.
Steps to reproduce:
Try to create a page that doesn't exist yet on testwiki (or another wiki on wmf.30)
Since the page doesn't exist, no editingold warning is shown
Warning is shown
Does not occur when creating a new section or editing an existing page
Relevant logic in EditPage::showHeader
I caused this in https://gerrit.wikimedia.org/r/#/q/Ib8f399e451c3a9b942001b48ea9a138f405c894c as part of T246284: Hard deprecate the Revision class