Eel and pickled plums are a bad pairing

There's a superstition that grilled eel (unagi) and the strong acidity of the pickled plums (umeboshi) react with each other and cause indigestion.

Verity:

On the contrary, the sourness of the salt-cured plums aids digestion of the oil-rich eel. They also happen to make this classic summertime food taste well together.

However, if you're in a dirty eatery and offered pickled plums with your eel, beware; it might be for the salty plums to disguise rotten eel.