In case anyone else hits this (admittedly very specific) problem, nathyong (https://news.ycombinator.com/item?id=11461326 )'s suggestion, and a subsequent USE (http://unix.stackexchange.com/questions/276731/shell-eating-...) question, made it clear that my issue is that I had some time turned on `nullglob`, so `https://www.youtube.com/watch?v=wbKJt1NQtZE ` was being taken as a very strange, certainly non-matching, glob.

Very strange. Perhaps it's some setup- or platform-dependent thing; I will file an issue. Thanks for checking!

EDIT: Very strange; I thought I'd give a try to what seemed a throwaway comment in your post, about quoting the string, and that fixed it. Thanks!

Without the double quotes (which seemed to be unnecessary for you), I still get:

    $ youtube-dl https://www.youtube.com/watch?v=wbKJt1NQtZE
    Usage: youtube-dl [OPTIONS] URL [URL...]

    youtube-dl: error: You must provide at least one URL.
    Type youtube-dl --help to see a list of all options.

I guess that some alias is grabbing some part of the URL string.

It's possible your shell has some kind of regular expression magic enabled that is eating the '?' and the '=' characters:

  $ echo youtube-dl https://www.youtube.com/watch\?v\=wbKJt1NQtZE
  youtube-dl https://www.youtube.com/watch?v=wbKJt1NQtZE
  
  $ echo youtube-dl https://www.youtube.com/watch?v=wbKJt1NQtZE
  zsh: no matches found: https://www.youtube.com/watch?v=wbKJt1NQtZE

Interesting! It seems that my `echo` is eating any substrings involving `?`.

    $ bash --version
    GNU bash, version 4.3.42(1)-release (x86_64-apple-darwin13.4.0)
    Copyright (C) 2013 Free Software Foundation, Inc.
    License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>

    This is free software; you are free to change and redistribute it.
    There is NO WARRANTY, to the extent permitted by law.

    $ echo a ? = =?
    a =