Why is Palpatine upfront with Anakin about an apprentice betraying his master?
I mean, it's not really a betrayal, if you have to tell someone to backstab you.
Wouldn't it make more sense to lie about it, pretend loyalty is paramount, and see who figures out for themselves what's what?
Also, does Palpatine WANT to be knifed in the back someday? Are Sith masters desperately praying to the dark side that their head ends up on a pike, and proud if it happens?