I had this happen to me last week. I was traveling on an award ticket issued by CO that had segments on CO, NW, and DL. The CO and NW segments had no problems, but upon check-in the DL kiosk claimed to not be able to locate my reservation. The DL agent was able to find it, but said that although I had a reservation, there was no ticket in the record and asked me for a ticket number. I had lost my printed itinerary, and just by chance had my previous segment's NW boarding pass on me, which happened to have the ticket number on it. If I hadn't had it, I'm sure I could have called CO and gotten the ticket number, but it did catch me a bit off guard, and reinforced to me the wiseness of having all confirmation, PNR, and ticket numbers handy.