The problem in this bug report is clear: The function get_and_unpack_sets() in util.c calls the menu MENU_distmedium as long as the status is set to retry.

The solution is to run this menu once before actually getting into this loop, before actually writing the changes to disk (see the file install.c or upgrade.c for an example).
Though the user who is used to fetch its sources from more than one source will be confused about the changed order of the question, 99% of the people won't even notice it. 

http://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=17792