After years and years of telling people that saying, "If you think so-and-so, you've got another thing coming," was incorrect, I was finally forced by TheLady to research my theory. And, no surprise to me (dannye says humbly), I was right all along.

The correct phrase is, "If you think so-and-so, you've got another think coming." Sure, it's incorrect grammar, but it's a colloquialism. And a quite fine one, at that.

If you doubt me here, you can put this phrase in a good search engine and find many examples.

What do you mean, incorrect?

When I use a word, it means what I want it to mean. No more, and no less. - Humpty Dumpty. (Apologies if this is a mis-quote.)

When I say 'You've got another thing coming', I mean just that: the listener will be the recipient of a further intelligence, to the effect that all is not lovely in the garden, and they were plain good old fashioned wrong.

Damn, that got downvoted fast.

FWIW, I did two Google searches on these phrases ("thing" versus "think"). The first match on both searches consisted entirely of the following:

Here's a case in which eagerness to avoid error leads to error. The original expression is the last part of a deliberately ungrammatical joke: "If that's what you think, you've got another think coming."

So "think" is original, but "thing" is grammatical. Choose appropriately.

The alternate "thing" version gained steam in 1983 when British Heavy Metal mavens Judas Priest charted with You've Got Another Thing Comin' off Screaming for Vengeance. In the song's chorus, lead singer Rob Halford clearly says it as You've got another thing comin'

Perhaps the phrase is different in Sheffield, or perhaps the lads are rockers but not rocket scientists. Nonetheless, this song helped entrench "thing" in the popular psyche of the mid-80s.

Incidentally, may favourite part of the track comes near the very end, during the fade. Rob Halford is repeating the chorus during the fade out, and you can clearly hear him wearily toss "it's a long song" into the lyrics as the track winds up.

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.