12. Beijng Duck and Hoisin Sauce?
Although Beijing duck (北京烤鸭) may be served with hoisin sauce (海鲜酱) in some restaurants (mainly American), it is not traditional. Hoisin sauce is Cantonese, as is the word 'hoisin' (in Mandarin, it's 'haixian'). When the first Beijing duck restaurant opened in Beijing in the Ming dynasty some 600 years ago, Guangdong (home to Cantonese food) was several weeks or months away from what is now the capital and its cuisine hardly known to the northerners.
Beijing's oldest surviving duck restaurants, including Bianyifang (便宜坊), established in 1855 and Quanjude (全聚德), esbalished 1864, still to this day serve their ducks the traditional way - with tianmian sauce (甜麵醬) aka sweet bean sauce, sweet flour sauce or sweet wheat paste.
Tianmian Sauce
Now, I'm wondering if the confusion arose because hoisin and tianmian look similar and people were eating tianmian, but thinking it was hoisin. I don't know. Everywhere I have eaten Beijing duck here in China, it has come with tianmian sauce. The only substitute I have occasionally seen has been sweet plum sauce. Never hoisin.