These people over react then find out they messed things up worse and then disappear and will never claim responsibility or have to pay for their screw up. DDT for example - there was such a scare and now we are finding out that millions of people die from malaria that could be prevented. See www.junkscience.com/ddtfaq.html
These people can't recant or they will be out of a job.
Here are some of the things I remember at the moment they've cried wolf over.
Eggs are bad - - oops sorry they're good
Oatmeal - - same
Apples - - Alar oops again
Butter is bad, use margarine - - oops margarine is bad, use butter
Asbestos, take it out - - oops leave it in.
Pesticides are bad, use organic - - oops people are getting sick from organic because pesticides aren't being used.
I am sure there are plenty of other examples and that is why I am so skeptical of scare mongers who have cried wolf for so long.