I'm trying to check in online for a flight tomorrow (less than 24 hours away) and I get all the way through to the end, then when I try to print my boarding pass it gives me a screen which says "THIS IS NOT A BOARDING PASS. You will need to complete your check-in at the airport ticket counter. Reason: Your documention will need to be verified"

I just want to know if the seat I chose during online check-in (an exit) seat is mine, becaese when I view my reservation it says I've still got a seat at the back (row 29).

Also, why would my docs need to be verified at the airport?

I'm flying from the UK to the US, and have a British passport...

Thanks for any info,
sc_uk