Editing a document containing ==='''Foo'''=== there is no indication that something is 'wrong'.
This is technically a skin-specific issue (although it applies to all WMF skins, and browser-default CSS), and the bold can be seen in a TOC.
That said we should help people avoid creating such content (e.g. on rich-text paste) and identify it so it can be fixed.
Perhaps we can fix this with a skin stylesheet rule, e.g. .ve-surface > hX > b { color: fuchsia; }
Or more naturally, use font-weight: 900;, which will render text in "double-bold" (assuming your font supports it). Firefox's default styles actually use that (and folks occasionally report bugs when they see double-bold text due to bold being used in a heading or table caption).
Or more naturally, use font-weight: 900;
This would work if you have mixed styling in == the '''heading''' ==, but would be harder to spot in an == '''all bold''' == headings.