In the function register_sidebars() in wp-includes/widgets.php the $args['name'] attribute is not filtered through sprintf(). This means that the following call to register_sidebars() will have unexpected results:
register_sidebars(2, array('name' => 'Foobar %d'));
Instead of producing two sidebars with the names Foobar 1 and Foobar 2 it will produce two sidebars with the same name: Foobar %d.
Secondly, multiple calls to register_sidebars() will fail if you do not specifically supply the $args['id'] attribute. This is because register_sidebars() maintains an internal counter that auto-generates an id for the specific sidebar (if one is not given as an argument). This counter re-starts at 1 on each call to register_sidebars(), effectively overwriting previously created sidebars on subsequent calls.
Example:
register_sidebars(2, array('name' => 'Foo %d'));
register_sidebars(2, array('name' => 'Bar %d'));
This will only create 2 sidebars - not 4. The two sidebars that will be created will be the ones from the last call: Bar %d
