For kicks, I just took a look at this old, forgotten issue.  I agree with the submitter that add_argument() should fail if nargs and metavar do not match, instead of having format_help() raise the exception later on.

I've attached a patch (issue9348_patch_v01.txt) with a proposed fix and associated test cases.  The drawback of my approach is that if a custom HelpFormatter is used which has different semantics for metavar it could cause add_argument() to incorrectly reject the metavar value.  I'm not sure how common that is though.