be in for a rude awakening

To have some very shocking and unpleasant or unwelcome truth (about something) due to be revealed to one. A lot of people were in for a rude awakening when the crappy high-interest mortgages they bought turned out to be worthless. If you think your computer's default antivirus software is able to deal with all the latest threats on the internet, then you're in for a rude awakening. Sarah thinks she's so smart, but she'll be in for a rude awakening when she starts college at Harvard next month.