Dr Suzanne Wuerthele is a toxicologist with the U.S. Environmental Protection Agency. Here she writes in a private capacity but clearly she knows what she's talking about.

The incident Dr Wuerthele is referring to involved a failure to follow containment rules with GM plants at a New Zealand research facility - see:
http://www.lobbywatch.org/archive2.asp?arcid=5495
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Dr Wuerthele's comments:

This lab incident in NZ is illuminating, like Starlink, Bt10, and U.S. pharmcrop contamination incidents before it. But will we ever learn?

Humans being what we are, intentional as well as accidental violations of rules are to be expected. This is why we have traffic cops, regulatory agencies, and jails. So, risk assessment must build into it the assumption that rules will not always be followed, and attempt to predict the outcome. Decision-makers have to ensure that simple rule breaches cannot create an unacceptable outcome.

This is what makes genetic engineering of food crops to make drugs and industrial chemicals so disturbing. There will be mistakes. There will be accidents. And there will be intentional violations. The US Department of Agriculture has mandated procedures which it believes will prevent contamination of the food supply, and it is so confident in them that it does not monitor to see if those procedures are working. That certainly looks like a mistake.

But even if their procedures really work, humans being what we are, it is only a matter of time before such crops are contaminated with biologically active chemicals never intended to be in the food supply.