DDT:
The problem with DDT wasn't so much the product itself- there are plenty of very hazardous chemicals around- but that it was put into the hands of consumers who used it everywhere, for every little pest problem they saw. For a long time, there was simply no getting away from it.
Today, there is some discussion of permitting DDT use again on certain pests that are highly resistant to other insecticides, but this time, by licenced professional pest-control firms only. New York City has a huge bedbug problem, with five-star hotels just as prone to infestation as homeless shelters. Present means to eradicate the critters are having limited success.
I would not advocate the reintroduction of DDT as a consumer product- no way. But for very limited use when all else fails, it seems to have a place.