Grrrr . . .
The problem with the "thing" misusage is that it's not only ugly and pointless, but very nearly nonsensical as well. That doesn't seem like a big deal for most people these days (if indeed it ever did, which I doubt), but it's not wrong to make sense, is it? Not morally wrong, anyhow. I mean, making sense is at least forgivable, right? In some cases? Extenuating circumstances and all that?
The first half of the sentence is implied; we'll italicize that and tuck it away demurely between parentheses:
"(If you think foo,) you've got another think coming."
Okay. We've got this "think" thing going on here. It's a sort of a motif. We work with it. You can change "think" to "thing" in both cases and end up with total gibberish, but is it really worth the effort? Maybe so. That's not for me to say. If we bite the bullet and leave it intact, the ungrammatical second "think" adds charm. It's blatant enough to be ironic. The point is that if we don't break it, we've got a very pleasing little chunk of language there.
Why break it?
Give me one good reason to break the damn thing, other than the sheer joy of mindless destruction for its own sake. Or simple drooling idiocy, if that's the way you happen to swing.