A true feminized seed can not become a male, because it has two X chromosomes. Usually you take a female, spray colloidal silver on the plant during early to mid flowering, wait for it to go hermi, take the pollen, destroy the plant that was sprayed, and apply the pollen to another female plant. Since each female only carries an X chromosome. the resulting seed is XX which is a guaranteed female.
There is another way to create a feminized seed and that is to allow the plant to stay in flower a few weeks longer then usually. This can force a female plant to hermi naturally, instead of you having to spray your plants. Once hermied, the process is the same as above.
Having said all this, your still dealing with humans and it's always possible that mistakes happen during a grow. A rouge male plant might not be found and culled in time or seeds might get mixed up during packing. That's always possible.