Board Thread:General Discussion/@comment-74.111.40.75-20130418102626/@comment-1290942-20130802172744

Ahsoka's existence doesn't necessarily conflict with canon; according to The Jedi Path (also not strictly speaking canon, of course) and much of what's written in the expanded canon of the novels, a Jedi Knight may (not must) be assigned a Padawan, but successfully training a Padawan to the rank of Knight themselves is a prerequisite to attaining the rank of Jedi Master; thus, Anakin's failure to attain the rank is not at all surprising, since his Padawan abandoned the Order.

Incidentally her abandonment of her studies actually makes her fit better into the canon than she otherwise would have; leaving Ahsoka's fate a mystery fixes a lot of the retcon issues her existence created. Weird things - like making her a Senator (as if Vader and the Emperor would have let her live if she had been so easy to get to like that) or trying to shoehorn her into Luke's life just make matters nonsensical and convoluted.