I remember way back in the day, way before the UA/CO merger on UA, flying ORD-LGA (I think - definitely somewhere domestic) and getting served Tropicana when I asked for some OJ. I knew that wasn't normal and asked why they gave us the good stuff, and they explained the inbound had come from YYZ, and so had it catered there (I don't even think they cater out of YYZ anymore, so yeah, was a while ago).