You’ve Got Another Thing Coming
“Out there is a fortune waitin’ to be had
If you think I’ll let it go, you’re mad
You’ve got another thing comin’”
Who are we to disagree with Judas Priest? Those are the timeless words of Rob Halford. Nevertheless, we shall debate since the correct way to say this is "you've got another thing coming." That is the proverb that is listed in the Oxford English Dictionary and the majority of other dictionaries, yet it is fair to say that the alternate version with the word "thing" has gained more traction, in large part because of Judas Priest's metal hymn from 1982.
The expression is a fantastic illustration of a "eggcorn," a language error in which people substitute a misheard word with one they believe makes sense in the context of the phrase. These inaccurate sayings are among the most common ones you can find, and since there is some rationale behind the changeover, it is probable that both versions will endure for the foreseeable future.