Wow, this debate helps me understand the meaning of "you've got another think coming."
Thank you guys for all this information.
I personally think that "think" is correct. People might think "thing" is right just because both "think" and "thing" sound the same and they have heard it since they were young. If that is the case, they just misheard it. It's hard to say thing version is right just because of this reason. I'm Korean. There's many proverbs here as well. And there's are some words I misheard it and used it incorrectly. But then I realized later that I misheard it.
In my opinion, we have to know and use correct expressions even though we might use wrong one in conversation. Thank you for reading.