I heard humors about banana peels having brown to black patches may cause cancer. The more spots it gets the more dangerous it becomes.
Authenticity of the following could not be guaranteed.
Banana peel is not dangerous and indeed has good effects. It is good for the eye and helps fight depression.
Boiling banana peels and drinking the cooled water is recommended. It is reach in serotonin and lutein. Serotonin is a mood balancing chemical that eases depression. Lutein is an antioxidant that prevents eye damage.
The flesh has the anti-cancer effect, not the peel.
Japanese professor said, yellow skin bananas with dark spots are eight times more effective in fighting cancer than green skin varieties. Bananas produces the substance called TNF, tumor necrosis factor. The riper the banana, the more TNF it produce and the more effective it becomes.
Further reading is not necessary as other articles are pointing to the same Japanese study.
In postharvest point of view. Brown spots in banana peels may indicate occurrence of disease, but not necessarily rendering it unfit for human consumption. On the other hand, bananas in perfect condition will still produce brown spots as it ripens. It increase as the fruit ripens until it reaches the full brown, saggy and inedible stage.
update: Eating Overripe Bananas