This is bound to be just the kind of news that genetically modified food opponents will exploit, while also causing worries for U.S. wheat exporters.

A farmer in Oregon recently found genetically modified wheat that Monsanto Co. had designed to be immune to the company's Roundup weed killer. The trouble is that the wheat was never approved for commercial use, according to the U.S. Department of Agriculture Department, and Monsanto stopped its development efforts eight years ago amid opposition from farmers, environmentalists and consumers.

The discovery was made accidentally when the farmer used the pesticide to kill wheat, and some of the plants survived. Those plants were traced to an experimental product that was tested in 16 states from 1998 to 2005. The Agriculture Department said it is investigating how the Roundup-resistant wheat wound up in the Oregon field so long after the testing ended. No country has approved production of GM wheat.

For those opposed to GM foods, this is sure to provide fodder for more claims about their ill-effects on human health (none of which are scientifically proven) and the environment (possible and worthy of more research).

This episode is also a reminder that it may be difficult to grow biotech crops without spillover to non-GM food. Altered food may find its way to the plates of those who want to avoid them. Never mind that many of the non-GM fruits and vegetables we eat had their genetic makeup manipulated through exposure to radiation or toxic chemicals.

The Food and Drug Administration's line concerning GM foods is that they are as safe as their non-GM counterparts.

The potentially bigger concern is the response of countries that import U.S. wheat and have stricter regulations on use and distribution of GM foods. Earlier today, Japan, the largest market for U.S. wheat, canceled an order. Because half of the U.S. wheat crop is exported, overseas demand plays a large role in setting prices.

It's no surprise then that wheat prices fell almost 2 percent earlier today. For those who grow the crop, the answer to how GM wheat ended up in that farmer's field can't come soon enough.

(James Greiff is a member of Bloomberg View’s editorial board. Follow him on Twitter.)