Another feature of the new webcheck-in application is that it reads your PNR more thoroughly than the old version, and there are certain things that will issue an Airport Service Document rather than a boarding pass. Wheelchair requests, Unaccompanied Minors, and undocumented or paper cert upgrades.

An Airport Service Document will NOT get you through security, and has to be handled at the customer service counter.

Anyone using a paper cert upgrade thatcan be booked ahead of time (gold Guest, NW elite upgrades, Consumer Affairs upgrades) needs to allow some time out front to turn it in, because now, they'll be consistently collected before a boarding pass is issued.