At en.wp, Fram provided a description of how to "trick" VE into showing some markup without actually typing it.
<<Start from an "empty" page (it contains some hidden comments, but since you can't remove these with VE...): [https://en.wikipedia.org/w/index.php?title=User:Fram/sandbox&oldid=593249688].
Start a simple article in the normal way, with a header and some text.[https://en.wikipedia.org/w/index.php?title=User%3AFram%2Fsandbox&diff=593249716&oldid=593249688]
Now reopen that page in VE: [https://en.wikipedia.org/w/index.php?title=User:Fram/sandbox&oldid=593249716&veaction=edit] Hey presto, I get <nowiki>== Test ==</nowiki> instead of the actual header appearance, due to the invisible and unremovable hidden comments. >>
I was able to reproduce this, see https://en.wikipedia.org/w/index.php?title=User%3AElitre_%28WMF%29%2FSandbox&diff=593889873&oldid=593889831 .
Version: unspecified
Severity: major