From New Scientist: GM crop mishaps unite friends and foes:

Friends and foes of the use of genetic engineering in US agriculture have united in criticising two accidents in which a food crop [soybeans] was contaminated by a [corn] crop from the previous year designed to yield pharmaceutical products.
[...]
"What if the GM corn had come up inside a corn field, instead of a soybean field?" he asks. "It could have cross pollinated and you'd have no idea where it was."