Assuming following:
1. optional argument, say "--foo", in a subparser;
2. at least two different optional arguments in the main parser prefixed with "--foo", say "--foo1" and "--foo2";

parsing fails with "error: ambiguous option: --foo could match --foo1, --foo2".

It seems like argument abbreviation mechanism described at http://docs.python.org/library/argparse.html#argument-abbreviations is not working properly when in use with subparsers...