Driving a US government vehicle into Canada without permission from either side is (I was later told) formally an act of invasion.

The government vehicle in question was the 15-passenger Ford Club Wagon that carried our AmeriCorps team around. We tended to commandeer it for entertainment on weekends. One weekend in early 1999, our entertainment was a trip to Vancouver. We got some funny looks at the border, but the Canucks were really very laid back about the whole invasion thing.