1. Open any commit for viewing in 1st browser tab, e.g. https://secure.phabricator.com/rP614f911217cf3e6e67d05f370d424147462cfa1a and click Edit Commit link on the right side.
2. Open another commit for viewing in 2nd browser tab, e.g. https://secure.phabricator.com/rPed2a5a9a34a73804f8349f891e7602661d7cb70a and click Edit Commit link on the right side.
3. note that both edit forms look exactly the same and commit sha in navigation bar belongs to 1st edited commit
4. when you save a form you'll be redirected to 1st edited commit page

The commit identifier (for git repos it's sha) from URL is just ignored.