I get the encounter, where a student comes to me and asks me to help him with his ward spell. But I haven't had this one yet.

i had this only once as i was lvling up my illusion spells in whiterun.

Happened to me once in Riften, occurs when you use spells or if one of your magic skills is moderately high (not sure), I had thought you'd get the same challenge from someone who uses a weapon, aint happened to me yet so not sure if it exists, need to check the guide book.