Generally, You Taught Me That is a Stock Phrase
directed at a mentor. The pupil is exhibiting, and thankful for, a skill or a value that the mentor taught him, and wants the mentor to be proud of him
In a more dramatic use, You Taught Me That is directed at a former mentor, in a situation where the mentor and the pupil find themselves on opposing sides. It could mean one of three things:
- The student defeats the mentor using a trick that the mentor had taught him.
- The student is sticking to the values that the mentor had taught him, before the mentor's Face Heel Turn. In this case, the mentor is a Broken Pedestal.
- The mentor had been a bad example to the student, usually unintentionally, since the mentor's actions didn't seem to live up to his words. This is what prompted the student's Face Heel Turn, and he is currently exhibiting the same flawed behaviour that his mentor regrets. Expect a lot of guilt on the mentor's part.
- An odd variation in that the "mentor" is consistently villainous: in the film of Harry Potter and the Order of the Phoenix (as in the book), Professor Umbridge makes Harry magically carve the words "I must not tell lies" on the back of his hand as a punishment for telling a Cassandra Truth (that Voldemort has returned). Towards the end, he and Hermione have lured Umbridge to a centaur herd, and when Umbridge pleas for Harry to tell the centaurs she means them no harm, he replies "I'm sorry, Professor, but I must not tell lies", while holding up the scarred hand.
- Harry does it again in the seventh film.
- In Mort, an Evil Vizier has a plan to poison his emperor but is outmaneuvered by the emperor and has to eat the poison himself. As he's dying he compliments the Emperor on what he's done and the Emperor says "I had a great teacher."
- At the end of Law Abiding Citizen, the villain is finally cornered by the protagonist, a prosecutor who is responsible for starting the whole thing by making a deal with a murderer. When the villain tries to negotiate, the prosecutor says he doesn't make deals with murderers anymore and references this trope.