[JadeNB]

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.

[nathyong]

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

[JadeNB]

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 =