He said he was in his corn field.

I don't think even Monsanto is stupid enough to go spraying r-up on fields "to see if something illegal is going on," although I suppose I would be tickled to have them spray a few hundred acres of our beans, preferably in a drought year. I'd say there would be a pretty good settlement involved if someone DELIBERATELY killed my soybeans by spraying them with no authority to do so.

Back to the original question: I'd probably lean toward it being a mistake. I know quite a few people who have been spraying fungicide on some of their corn have been putting a swath across fields they didn't spray for educational purposes. It's not too hard for me to believe a pilot could accidentally hit the wrong field. I'd think that he would have noticed a pickup parked at the edge of the field and skipped it though.  

Charging booms or unloading over an unknown field would not be acceptable.  That amounts to a deliberate act and I doubt any applicator would do it.  Making a mistake and hitting a field in the wrong section is just that, a mistake.  Fortunately there was probably no harm done.