Mariel Martinez asked Google's Gary Illyes if having an incorrect hreflang implementation will hurt you or not and Gary said no, it won't hurt you.
Gary responded on Twitter saying "nope," it won't hurt you but Google will "just ignore it."
Here is the set of tweets:
@marieldoesseo nope, we'll just ignore it
— Gary Illyes (@methode) August 1, 2015
I personally also thought there is room to cause serious damage when implementing these incorrectly but I guess not?
Google recently started sending hreflang notifications for incorrect configuration via the Google Search Console. So these questions are more relevant now than ever.
Forum discussion at Twitter.