There are many strains of the cannabis plant with extremely stable genetics.
Several things can cause your cannabis plant to get shocked and become hermaphroditic, such as light leak during the flowering stages of the light cycle.
Sometimes the cannabis plant will produce male organs on a female plant.
This can infrequently happen with your feminized cannabis seeds.
You can also take a good digital photo and enlarge it until you see the trichromes.
Often the sex organs "bananas" do not open on feminized cannabis plants but you should treat them as if they are open and take care not to bring that pollen back in to the females.
Severe variations in the PH levels or fertilizer can also cause the plants to create male sex organs. Watering incorrectly can shock your plants as well as the introduction of hormones improperly or at the wrong time.
Growing past the recommended harvest time can also cause hermaphrodites.
If you are going to change something in the grow process it is usually best to do it slowly if you are growing indoors this is much easier.
Outdoor grows should not have any problems with hermaphrodites but the climate cannot be controlled as exactly as indoors.