But right at this moment in time (which is for basically the last several months), I have not seen anyone use the metaphor "rein in" correctly. So I'm going to waste my breath _once_ and then never mention it again.
To Reign is to rule or hold royal office.
To Rein In is to pull on the leathers attached to a bit (generally metal) to control an animal, generally a horse.
And now I will stop, because even I can tell that everyone else has ranted about this already, to no good effect. So I will rein in my impulse to reign in the realm of grammar, and just go back to tolerating all the phonetically based confusion.
It's not like I don't produce egregious errors of my own, all the time, but especially when attempting to blog while a child is climbing on me.