There are little things that tend to break proper visual consistencies in Ubuntu, such as various opened-in-phases menus, like for instance, the Network indicator menu, that was producing a flicker (due to its internal refresh) when opened.
This minor issues tend to create an unpleasant visual contrast with the regular, fast menus.
Ubuntu 12.04 LTS addressed, weeks ago, the Network indicator's flicker issue, nevertheless, Firefox and Thunderbird were "featuring" a similar behavior when opening menus.