They harass you for a quite a while and detain your for possibly a day or more. They might even play a game of 'I don't think your actually a US citizen'. You

They harass you for a quite a while and detain your for possibly a day or more. They might even play a game of 'I don't think your actually a US citizen'. You also get a note of 'had problems at the border' and all future crossings for you and anyone travelling with you will be far more unpleasant for many years. Not very pleasant for most people, and most people don't care about the contents of their phone that much unfortunately.