In an unfinished and not-ready-for-publication post, I tried to insert a link from the toolbar of the SE-internal editor. Unsatisfied with the result, I wanted to just cancel the dialog. That was "canceled" in that the content of the link box didn't appear in the post, but it also submitted the post prematurely.
This seems to have happened more than five times now to me and appears as a somewhat reproducible bug. It does not always happen, but just now it happened one time too many.
Browser used is Safari 12.1.1