When the float was defined on the same page you might end up with the first
part of the supertabular on a page by its own.
When the list of unprocessed floats is not empty it will be made empty by the
first part of the supertabular (if it is split across pages that is) because it
calls \clearpage
.
\vbox
messages. These instances are described in the (documented version of) the
style file itself.