Looking a bit more at the examples in the "nargs" section of the argparse docs, and on the rest of that page, I think I see the intended usage pattern emerging. nargs='*' is only ever used on that page with an optional (by prefix) option, or with the last positional defined. Conversely, nargs='+' (or "+") is only used with a positional or with an optional that's paired with action="extend". 

This makes sense given the 0-or-more vs. 1-or-more distinction, but could it be enforced by exception or by warning? Specifically, I can think of a couple dangerous (as far as unintended consequences) cases:

1. Define a sequence of positionals with a nargs='*' sandwiched somewhere in the middle. Then passing fewer arguments at the command-line than defined positionals will cause the nargs='*' one to be skipped, rather than populating truly in order. Example:

def _parse_cmdl(cmdl):
    parser = argparse.ArgumentParser()
    parser.add_argument("outdata", help="Path to output data file")
    parser.add_argument("plotTimes", nargs='*', help="Times to plot")
    parser.add_argument("outplot", help="Path to output plot file")
    return parser.parse_args(cmdl)

would result in a parse of something like:
$ ./tinytest.py a b
outdata: a
plotTimes: []
outplot: b

2. Case initially presented, i.e. a nargs='+' with a hyphen-prefixed option name. If the semantics are no different than for nargs='*', could a warning or exception be thrown for defining something this way? It would feel safer to not have the meaning of a value like this to nargs not be conditional on the name of the option.