Back in January, @realscientists (I believe it was @upulie at the time) was talking about nanotechnology and mentioned the growing knowledge about hazards:

For example, it was only some time after we started working with carbon nanotubes that it became apparent that we should introduce protocols for working with them. This is something we should think about for a lot of our research

To which I replied:

It was only some time after we started working with _____ that it became apparent that we should introduce safety protocols.

In that blank, insert whatever amazing new thing (not just chemicals) that is being hailed as a breakthrough. For starters, consider cyanide, arsenic, lead, and radium, materials that were in common use for their wonderful properties, but that turned out to have toxic consequences, as showcased in the recent American Experience episode based on Deborah Blum's The Poisoner's Handbook.