Without careful study, plants with undesirable characteristics may be released into the global ecosystem. How a gene affects a plant depends upon the environment. The same gene can have different effects, depending on the environment in which the new plant is growing. What appears predictable and safe after a few years of observation of a small test plot may turn out to have quite different consequences when introduced into millions of acres of croplands in the U.S. and elsewhere, where conditions vary widely. Only time will tell.
Genes can travel to nearby, related plants on their own.
This is called gene flow. In 1996 gene flow was discovered to be much more common that previously thought. According to Science magazine, many ecologists say it is only a matter of time before an engineered gene makes the leap to a weedy species, this creating a new weed or invigorating an old one. A public health disaster was narrowly averted in 1996 when a group of researchers tried to improve soybeans by giving them a gene from the Brazil nut. The gene from the Brazil nut was successfully transferred to soybeans.