I'm sure there have been many threads like this, but one of them was mine. We were flying Airtran with our (at the time) 4 year old son and 23 month old daughter. We intentionally scheduled the trip before she turned 2 so we wouldn't have to buy a seat for her (when we knew she would want to be in our laps anyway). The counter agent didn't believe us and we didn't have her birth certificate, so she made us buy a ticket (which cost about us much as the other three combined). One flight attendant insisted that we put her in the seat instead of our lap. Then another flight attendant chewed us out and said she could be fired for allowing a 2-year old sit in a seat. We got a refund when we faxed in the birth certificate after we returned, but it was quite a hassle. Now we bring both of the kids' birth certificates everywhere we go.