No, I am familiar with abrogations. Not the kind of thing I would expect from an unchanging, infallible God though.

Muslims often say that if a hadith contradicts the Qur'an then it is false, because Muhammad never contradicted the Qur'an (according to them) - maybe that is what you are thinking of.
This is often used by those who want to reject nasty hadith.
Yes, I think you are right.