I'm also planning a trip to Cuba for spring break. As a US citizen living in Canada, I don't see any problems--I'm "supposed" to be in Canada in the first place, so there won't be any questions there. Many times, US citizens coming through Canada in January will go directly home and go through US-preclearance in Canadian airports, tan and wearing summer clothes--highly suspicious to immigration officers. As I won't be going to the US for some time after returning from Cuba, I don't see how they can figure anything out.

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regards,
brian