procrastinaters.com

The answer to this one actually took me by surprise. I was sure it was 'thing' but a little research shows it can be both, and actually 'think' is more commonly used. The term itself is correct either way and it's meaning doesn't really change despite the meaning of the words.

'You've got another think coming' really means that you thought one thing and are now being told that you're wrong and need to rethink your previous thought to come up with the correct conclusion.

'You've got another thing coming' is most likely the malformation here but both phrases easily date back to the early 1900s. It's the same thing tho. You think you've thought of the correct thing, but the thing is wrong, so you need to think of another thing.

My vote still goes to thing. It rolls off the tongue better, sounds more modern and most importantly- who the hell am I to argue with the godfathers of modern metal?

"If you think I'll sit around as the world goes by
You're thinkin' like a fool cause it's a case of do or die
Out there is a fortune waitin' to be had
You think I'll let it go you're mad
You've got another thing comin'"

No! It's another thing coming. You think this - but there is another thing coming(down the line) to make you change you're mind.
You got another thing coming!