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.