I heard a couple of guys on the radio tonight rebuke Bill Hayden (in his absence) for using the phrase "if he/she/it thinks blah blah they've got another thing coming"...as opposed to "another think coming". I nodded in agreement as I 'knew' they were correct. A few minutes later they read an email from a listener asserting they in fact were a couple of dunces and incorrect, and that Hayden's usage was right. Of course, they started googling and came back with the magic words (for me, at least, when grammar and spelling disputes are involved) - OED (Oxford English Dictionary) - which supported their view (and mine) that think is in fact correct. And after a bit of googling myself I am satisfied that this version, which is what I have used 'forever' (in fact I had no idea an alternate version even existed) is the right one. I just hope this is not another case where so many people get it wrong, it becomes 'right', as happens with certain words and phrases. Evolution of the language is one thing, evolution through ignorance is quite another.
PS. I see both versions have had a run on AFF, though not, as far as I can tell, in this thread.