How could genetic contamination happen?

Genetic contamination happens in one of three ways:
1. Through cross-pollination – wind or insects can carry pollen over large distances. E.g. Genetically engineered corn could contaminate corn being grown on farms or in backyard gardens.
2. Mechanical contamination – through mixing of crops in storage, plant material left on farm equipment etc.
3. Through the passing of DNA from plant material into the soil where it can be picked up by soil bacteria.