This post by David Zaruk at the Risk-Monger blog gives a number of examples illustrating that precaution isn't always the best strategy. Sometimes the precautionary principle is invoked as a reason to avoid taking an action. In other instances it is invoked as a reason to take an action. "We've got to do something" in the face of some problem, and "trying something" in the face of uncertainty is the taken as more cautious than doing nothing. David writes:
“During the Great Plague of London (1665-66), the authorities were convinced that the outbreak of bubonic plague was being spread by cats. As cats had then been looked upon by religious leaders as symbols of evil and witchcraft, the crisis created the perfect opportunity for zealots to purge London of this feline scourge. The local authorities had no evidence that the cats were spreading the plague, but via the virtue of precaution, they could be seen to be acting in a time of panic. Resisting public pressure from vocal zealots was not politically expedient, and in any case, who would really care if a few thousand cats were tossed in the Thames. Well the rats thought this was just fine, and as the rat population exploded, so too, obviously, did the plague (spread via rat fleas).”
He goes on to point out the unintended effects of modern day precautionary actions. These typically come about because of a failure to think on the margin and to consider behavioral response. If pesticide X is banned, that doesn't mean farmers use no pesticide. They switch to pesticide Y. Thus, the better question is how X compares to Y. Another questions rarely asked: if X is banned, then what new mix crops will farmers grow instead, and what environmental, health, or fiscal effects will that have?
David concludes:
“Precaution is the perfect tool – it worked for the zealots in medieval times and it still works today.
Note that I am not equating the use of the precautionary principle with medieval mind-sets. Precaution is a basic human reaction (no one willingly wants to hurt themselves). But the use of precaution as a regulatory tool by eco-religious zealots to spread fear in order to promote some medieval-inspired conception of agriculture and a communal-based economy regardless of evidence or the negative consequences is not only irrational, but also morally indefensible.”
(Note: I don't necessarily agree with all the hyperbole in the piece, particularly the last sentence)