Likewise, Scotland's national dish, haggis, is also banned, and not just because it's a meat product, which always gets the attention of Customs. Haggis is created by pushing a sheep's heart, liver, and lungs into its stomach and then simmering the result until it resembles food. US Customs objects to this not out of culinary discretion, but because it contains lung, an item whose importation has been banned for sanitary reasons since 1971.
The restrictions on haggis and ackee are fairly well known, at least in their home countries, but others are nearly impossible to know about until it's too late. The most surprising items may be Kinder Surprise eggs, the famous Swiss sweets with little toys in the centre. Believe it or not, Kinder Surprise is specifically banned by US Customs, thanks to a 1938 food safety law that illegalised the sale of any confectionery containing 'non-nutritive' items. Surprise, indeed.