When I hit the "edit source" by accident, I can just press ESC and I'll stay exactly in the page where I am, without changing anything (not even scrolling the page). However, if I accidentally click on "edit", I can't stop the process which loads VisualEditor.
I reminded this problem while reading comments on T102398: Migrate wikis to use a single edit tab which has both visual and wikitext modes and allows on-the-fly switching between them.