The possibility for the user to edit a content is only checked when the 'edit' menu item is created. If the item is locked by another person, the lock icon is painted red and all the items which exclude mutual access are disabled.
For example, the classes EditContentAction (present in the anycontent plugin, in the structured plugin, etc.) don't check if the content is already locked, so the user gets no exception or message if he enters explicitly the corresponding content/edit/xxx URL, or if he has not refreshed the content page and another user has begun to edit it meanwhile.