have another thing coming

To have wrong or unreasonable expectations; to have to change or rethink one's plan or idea. Typically preceded by "If (someone) thinks…." This is a common variant of the original phrase "you've got another think coming." If you think you can walk in here and just demand a raise, you've got another thing coming. If he thinks he can hit it big without paying his dues, he's got another thing coming. If you think I'm going to lend you any more money, you have another thing coming!

you've got another thing coming

Your expectations are wrong and unreasonable; you better think again. Almost always preceded by "If you think (something)." This is a common variant of the original phrase "you've got another think coming." If you think you can walk in here and just demand a raise, you've got another thing coming. If he thinks he can hit it big without paying his dues, he's got another thing coming.