Full Idea
It is now recognised that the apriori and the necessary don't always have to go together, ..and Kripke has suggested examples of necessary-aposteriori and contingent-apriori beliefs.
Gist of Idea
Kripke separates necessary and a priori, proposing necessary a posteriori and contingent a priori examples
Source
report of Saul A. Kripke (Naming and Necessity lectures [1970]) by Paul O'Grady - Relativism Ch.4
Book Reference
O'Grady,Paul: 'Relativism' [Acumen 2002], p.119
A Reaction
The simple point is that whether something is necessary or contingent is a quite separate question from how we come to know that they are. There isn't a new mode of reality called 'necessary a posteriori'.