Supreme Master Ching Hai told us that once her great master and his disciples saw a living snake, from whose body many worms came out and ate the snake's meat and eyes. Her great master explained about the karmic relations between the snake and the worms. They used to be Master and disciples in their former life, but the false master prevented the disciples from liberation. That's why his disciples came and revenged him in this lifetime. The Supreme Master reminds us that before we become a master, we shouldn't teach others casually, because the consequences would be incredible.
To be a master, we should have enough power to digest the karma from all sentient beings. If we haven't got much merit yet or reached a very high level, we shouldn't go to teach others; otherwise, it will be harmful to both ourselves and others. If we teach others with ego, that would be even worse; we have to share not only their karma, but also the karma of those invisible beings too. Even when great masters share the karma from all sentient beings, they will also have problems (not without problems), the problems will pass very quickly. As for those who do not have enough good karma or merit, not even for themselves, but they still go and help others, surely both will owe a debt.