I was editing this article just now, and experienced 2 edit conflicts during a single edit attempt. In both cases, I resolved the edit conflict manually, then first clicked View Changes, which worked as expected, and then clicked Save, after which I experienced this bug - a bogus "You have not provided an edit summary" message appeared and meant that I had to click the Save button again.

Please note that I have several user scripts enabled in my settings, which might be relevant. Also, I had originally used Visual Editor for the edit, but had to fall back to old-style editing to fix the edit conflict. That also might be relevant, I would speculate.