Being pulled over while they go through some sort of ownership check, or just tossing the whole truck because you don't have everything they normally expect. I'd like to avoid the whole inconvenience, personally.
So hypothetically...
Border Patrol - Do you have an ownership for that bike?
Me - No. It was a spare frame from Kawasaki Canada's race program. I got it through the dealership I used to work at. Here's the bill of sale for the frame from Inglis Cycle. There was no NVIS or title that came with it but the VIN is clean. Run it if you like.
Border Patrol - Ok. Thanks
Further, I have never been asked by customs (on either side) to see paperwork on any bike in tow.