have another thing coming

have another thing coming

(One's) expectations are wrong and unreasonable; one better think again. Almost always preceded by "If you think (something)." The original phrase is "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.
See also: another, coming, have, thing