Tabeawase...

Many Japanese avoid having unagi and umeboshi (pickled plum) at the same time. It is a superstition called "tabeawase" that certain combinations of food have bad effect when taken at the same time. A combination of tempura and watermelon is another example of tabeawase.

My mom used to have a book written by Chinese people on tabeawase taboo combinations. I thought that merely touching the book would poison me or make me sick. So my brother often chased me away with the back cover of the book...meanie, but I got him back years later. ;)