most people think that GM food  is as harmless as cross-breeding plants to get the desired characteristic.  This is not the case.  By inserting a gene sequence into an organism, you have absolutely no idea what will happen because gene sequences overlap, and genetics has just discovered that it takes two or more gene sequences to have a characteristic expressed, not just the one as we had always thought.

There's been a case where scientists wanted to introduce a gene sequence for vitamin A in a plant, but once the plant was mature, they realized that an unknown, poisonous compound was also being produced.

The testing of these GM products isn't extensive enough to rule things of this sort out because the mechanism is not yet fully understood.  

Also, with cross-pollination occurring in the wild, this can happen by accident.