Just heard last night about a Canadian/American flying to US for a conference denied boarding. She had only her US passport with her and was told she needed both. That makes sense to me as she would return to her home in Canada, but I was told that in the past she had come and gone with only the US passport though she lives in Canada.
I'm posting this as a PSA, but since we need a question--here it is: what's up with this?