Coming back from the 1000 last year we were stopped coming back across the border (Mexicali crossing) to the States. We have 2 five gallon cans in the back of the truck, both were bought in the States before we went down, had the reciept and everything but for some reason they still made us pull into the security check area. They left us there for a while, ran all our paperwork and then came back to us and told us to never bring fuel back into the States.
Really not a big deal other then I was sick (Damn Mexcio tacos) and was ready to explode, sitting there for an hour almost did me in as they would not let us out of the truck.