Someone suggested that you keep a few boarding passes for use under certain circumstances while renting a car.
Say you flew in on Juniper Air but want a mileage bonus on Oak Air when you rent your car. Just display an Oak Air boarding pass in your pocket when you get your rental car, so the counter attendant won't bother to ask for proof you flew in on Oak.
I do not vouch for this method, do not even know if it is ever needed...I just read about it some time back in some forum on FT.
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Middle_Seat
[This message has been edited by Middle_Seat (edited 08-21-2001).]