Mercury poisoning. https://en.wikipedia.org/wiki/Mercury_in_fish

Which has to be the leading contender for our civilization's equivalent to "those Roman Empire folks were so dumb for using lead, ha ha". Though it is highly doubtful that lead poisoning caused the downfall of the Roman Empire, one to two order of magnitude elevated levels of exposures compared to normal background levels were no picnic the bodies of for those exposed.

Another example of distributed costs, concentrated benefits really screwing us over in the long-run.