The New York Times reports on a farmer that found some grass he didn't want growing, tried to kill it with glyphosphate, and discovered that it was RoundUp-resistant GMO wheat:

He [Michael Firko, acting deputy administrator for biotechnology regulatory services in the Agriculture Department’s Animal and Plant Health Inspection Service] said it was not clear how the wheat got to the farmer’s field. The last field test of that type of wheat in Oregon was in 2001, he said.