I'm sure it must have happened, because people travelling backwards and forwards across the border would be carrying or incubating influenza, potentially measles, and perhaps even mumps. Across the border by a land conveyance, it's entirely possible that it could happen. People travelling from Nova Scotia to the rest of Canada have carried mumps with them. We've had Japanese tourists flying in recently developing measles when they got here, and we've quarantined 120 people in the party. So it's entirely conceivable that somebody who is incubating a disease could use a land conveyance to cross either an international border or a provincial or territorial border.
On May 30th, 2007. See this statement in context.