You could have spent a century testing CFCs in a lab environment. The problem they caused with the ozone layer would still not have become apparent until CFCs were used in the real world where they could interact with the ozone layer.

There is no amount of testing and preparation that can account for every possible outcome or interaction.

Asbestos is another good example. It is naturally occurring and quite common and was used as a building material for millennia. It is lightweight but strong, flexible in thin sheets, and fireproof. It's an extremely useful and versatile material, and abundantly available.

It wasn't until the 1900s that medical testing linked asbestos fibers to several health risks. It basically required the entire history of human development for our medical technology to identify the danger. No amount of testing, analysis or review done prior would have mattered.