Back in the early days of e-tickets I bought a ticket from a travel agent, which I thought was an e-ticket but turned out to be a paper ticket. I took the itinerary with me to the airport but left the rest of the materials in the envelope at home, not realizing what was inside.

Naturally enough I encountered the same problem. Not allowed to board the plane. I thought this was ridiculous, since of course the whole thing is in the computer! When I asked the agent why they couldn't just turn my ticket into an e-ticket, she said she could not... but the issuing TA could. So I called my TA and she punched some stuff into her computer while I was on the phone with her, turned the paper ticket into an e-ticket and then 5 minutes later I was back at the gate being issued a boarding pass.

This was a while back, pre-9/11, when you could go straight to the gate and check in there, so maybe things have changed. But if that rule still exists it is good to know in case others find themselves in that situation.