No in fact you do not. This is an Internet/activist myth.

victorbjorklund 2 hours ago | parent [-]

Source that it is legal to keep the profits and the plants from a patented crop that can’t be prove you have intentionally planted it there? As far as I understand Montosanto claims it would always belong to them no matter how the seed ended up there.

	▲	tptacek an hour ago \| parent \| next [-]
		Feel free to cite the case they've brought where they claim that! They have sued farmers for innocently acquiring their seeds (through the wind or whatever) and then spraying their crops with Roundup (ie: using the whole system).
	▲	HDThoreaun an hour ago \| parent \| prev [-]
		There is absolutely no case law suggesting it is illegal to harvest and keep accidentally cross contaminated seed. Seeing as farming seeds is default legal there would need to be precedent otherwise for such an act to be illegal.