This is a bit ugly, but sometimes the error while parsing an argument seems to take precedence over anything (including other errors) that might be encountered later:

$ date -R -R --help
date: multiple output formats specified
[$? = 1]

See also uutils/coreutils#4254 (comment) for further examples.

I propose changing the interface to:

pub trait Options<Arg: Arguments>: Sized {
    /// Apply a single argument to the options.
    fn apply(&mut self, arg: Arg) -> Result<(), Error>;

    // Rest basically unchanged; the default `parse` impl would simply call `parse()?` instead.
}

What do you think?