When VE is active on a page it inserts its own "edit" tab (veaction=edit) and renames the legacy edit tab (action=edit) to "Edit source".
Since VE is not active on all pages though the "edit" tab's functionality is inconsistent across pages (sometimes opening VE, sometimes opening the Wikitext editor).
VE should rename the legacy edit tab to "Edit source" on *every* page (even if it is not active and therefore not inserting it's own "edit" link so the button's functionality keeps consistent across pages.
This was also discussed on http://en.wikipedia.org/wiki/Wikipedia:Village_pump_%28technical%29#Change_the_.22Edit.22_tab_to_.22Edit_source.22_in_all_namespaces.
Please note (as discussed in the linked section) that renaming the legacy edit tab globally by simply changing the existing interface message is *not* an option. People not using the visual editor should not see "edit source" by default but should continue to see the same text that was always there.