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The assumption that the USA is America. I once told someone from the US that a another friend of mine was also from America - which they were, they were from Brazil. The US friend told me later "You know they aren't really American, they are South American". ''yes - I have to admit I was hoping to provoke such a reaction.
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The US/Canadian cultural assumptions. Though there is a cultural difference between US and Canada, this is not as large as the difference between these countries an South American countries.
A lot of folks "south of the border" use the term americano to refer to people from the US.
See also AmericanCulturalAssumption