Old post but to clarify that I have understood correctly the corruption rules in page 176 and the mystical power rules in page 119.
I must point out that the rules are not very clear and I would have preferred some reminders at the mystical powers (p. 130) and rituals (p. 140) about the corruption in learning these skills, but I assume it's 1 permanent corruption for a level.
So, let's say our character has resolute value of 15 (corruption treshold = 8) and he is a sorcerer.
He learns two mystical powers, Bend Will and Curse and a ritual of Possess. This gives him 3 points of permanent corruption.
He now uses one of the skills to Bend Will and after a succesful resolute roll, takes 1 point of temporary corruption.
Later in the act he uses a Curse and fails his resolute, taking full 4 points of temporary corruption. This already puts him in the treshold and he immediately gets D4 points (rolls 2) points of permanent corruption.
Now he has 5/8 points of permanent corruption and is in a very bad spot already.
Did I understand this correctly? If you have more than two mystical powers learned, you cannot "safely" cast more than one spell per scene if you are sorcerer.