I was in Paris last week and received the following: "Thank you for choosing Auto Check-in for your upcoming United Airlines flight. Unfortunately, we are unable to provide your boarding passes at this time.

Please complete your check-in at the airport on your day of departure. We apologize for this inconvenience."

I wasn't aware I'd chosen auto-check in, though I would have been fine with it. I checked in online a few minutes later, no problem (except for the vanishing upgrade list.)