[have] another thing coming

to say that someone's opinion or judgment is completely wrong

The idiom "have another thing coming" originated as a colloquial variation of the earlier expression "have another think coming." This phrase, in turn, emerged in the 19th century, likely in British English. Over time, it evolved into the current form, "have another thing coming," possibly due to the influence of regional dialects and usage. The expression is now used to convey a sense of correction or surprise to someone who holds a mistaken belief or expectation.

If you think this project will be easy, you have another thing coming when you face the challenges ahead.
She believes she can finish the assignment in an hour, but she has another thing coming once she realizes its complexity.
If you assume the task is simple, you have another thing coming when you see the detailed requirements.
He thinks he can win the competition without practicing, but he has another thing coming when he faces the skilled competitors.
If they expect us to agree without any discussion, they have another thing coming; we need to address our concerns first.