What was the hardest part of your internship that nobody prepared you for?

Elder millennial here who teaches MBA level students. I've been researching what actually determines whether interns get a return offer.

The themes that keep coming up:

* Managers want interns to take more initiative and manage up * Interns want clearer feedback and don't know how to ask for it * The "unwritten rules" of office life (how to dress, whether it's okay to have your phone out, general expectations type stuff)

For anyone who has had an internship recently, what do you wish someone had just told you? I'm trying to get a full picture of struggles from the intern and manager side of things (and as mentioned..... elder millennial so it's been a hot minute since my last internship)