I've always thought the expression was if someone says "if that's what you think, you have another think coming." But I am currently reading a young adult book which has the phrase as "you have another thing coming," and it is used two times. Have I been wrong all these years?
I'm in the other camp. I always heard and said thing, and I was unaware that think was out there. I know that Grant and Martha did discuss this, but I can't locate the program in which they did. To my recollection and horror, I believe Grant said that thing was a corruption of the original think in the expression. He did mention that thing was also very common.
I had almost forgiven him for this pronouncement, but now that you bring it up again, I have another think coming.
Indeed, it is. Thank you, Emmett.
I blame Judas Priest! (For the record, I've always used "thing", though "think" is appealing due to the wordplay.) Looks like Jan Freeman has also covered this topic.