I travelled to the USA a few years ago and inadvertently had both my Canadian and Finnish EU passports in my passport wallet. When I was taking my Canadian passport out, the US Customs Officer saw the Finnish EU passport and asked to see it too. I thought nothing of it. But then I was escorted to a secondary examination and asked why I had two passports. I told him why and he proceeded to tell me that the USA does not allow its citizens to hold two citizenships. And I was in my way.