The last thing I want to do on Christmas is start to read an Avatar fanfic That I think is decent for three chapters then get to the fourth chapter upon which Aang's deceased spirit posesses Zuko then puts Katara in a hypnotic state then rapes her (and by side-effect rapes Zuko too) with Zuko's body to implant some mystical Avatar semen so that she (who is 14, mind you) will be mother to the next Avatar.

God, yes. You're reading along, and then suddenly the writer/story goes bugfuck crazy and you taste bile. It's one of the most unattractive things about fanfic, imho. Published fiction does it sometimes, too, but its writers are usually more aware of (a) the actual moral implications of a given situation, and (b) the trust relationship between writer/reader.