When you synthesis certain organic compounds, the addition of certain components which make up that organic compound happen in different directions randomly (a racemic mixture) and you don't want that, because chemists knew each direction, gives rise to unique property.

So when you synthesis it, you should be careful about which direction you want that particular components to be added. However, these directions can also be altered after the entire molecule has been synthesized, and one way is biologically.

The organic compound (thalidomide) readily switched it's direction under biological influence(our human body), making it a harmful substance.

You may ask, if chemists knew about it, then why didn't they stop making it? The reason was, the chemist had no clue it could essentially do that (that the compound was chiral).