In the landmark case you are probably alluding to (Monsanto Canada Inc v Schmeiser), Monsanto Canada sued a farmer both for accidentally growing contaminated crops in 1997, and for using seeds from the contaminated crop in 1998. They dropped the charges regarding the 1997 case mid trial, probably because they wanted to set a clean precedent.
Note that winning the suit over the 1998 crop means they won a de facto ban on saving seed from one harvest to plant the next year -- it is beyond the means of independent farmers to detect contamination, or filter out contaminated seed from their own harvest. In addition to creating an economic burden (farmers must now buy seed), it also means selective breeding of crops is now extremely risky from a legal perspective.
From the Canadian supreme court's judgment:
"Thus a farmer whose field contains seed or plants originating from seed spilled into them, or blown as seed, in swaths from a neighbour's land or even growing from germination by pollen carried into his field from elsewhere by insects, birds, or by the wind, may own the seed or plants on his land even if he did not set about to plant them. He does not, however, own the right to the use of the patented gene, or of the seed or plant containing the patented gene or cell."
Here is a more recent case. Monsanto has sued 850 farmers over seed contamination: http://www.activistpost.com/2013/08/monsanto-can-sue-farmers...
Apparently 700 of them settled out of court, so documentation on that front is thin. On the other hand, the Schmeiser (from above) countersued Monsanto after the above judgement, for (among other things) libel, and monsanto eventually settled out of court.
Monsanto has promised not to sue for < 1% contamination, and the courts bound them to that standard. No one thinks it is feasible for small farmers to keep contamination levels below 1%. (Note x=>y doesnt imply y=>x, but if they aren't going to sue farmers for contamination, why not publicly document that policy instead of this 1% rule?)
Finally, organic farmers are repeatedly suing over contamination (which is at least as financially damaging as Monsanto coming after them), with mixed results.