What are the biggest scientific mistakes ever made?

The introduction of the cane toad to the sugar cane fields in Queensland Australia is a good one. they were suposed to get a beetle under cotrol but now they are killing native wildlife as for afield as the Northern Teritory and still spreading.

From RALFWHO:

• Similarly, the mongoose was introduced to Hawaii to control the rat population. Now they have a mongoose AND rat problem.
• X-RAY shoe-fitting equipment.
• Thalidomide.
• DDT
• Not the biggest, but only if ask others not afflicted: anal radium treatments.