The other day I was playing Banished and built a few fishing docks. I then took a look at the amount of fish it was producing and noticed that it was also producing 38 corn for that season. That never happened again but that led me to wonder, "Is this a bug or is this just some random chance type thing built into the game (Although I have no idea how that makes sense)".
Crop fields will, on occasion, pull nearby workers to help harvest their yield. This is because designated farmers will turn the crop into food baskets and, on occasion, leave them to go do something else (bring food/firewood back their house, go to sleep, etc.) At this point, the food basket becomes a resource just laying around and any nearby citizen will pick it up and take it to the relevant storage facility. Said resource will now count as being produced by the citizen's work facility, leading to your fishing docks producing corn and (in my case) mines producing wheat.