When Harding reads a copy of the Chronicle article which describes the death of Roger O'Neill, the byline shows both John Krajewski and Mattie Storin. Mattie Storin had been fired by the newspaper prior to these events, and so her name could not have appeared on that byline.
When Sarah and John talk about Mattie, John tells her that he doesn't know if the girl had an affair with Urquhart. But in the previous series "House of Cards", Mattie explicitly tells John about the affair.
When Harding and Krajewski are talking on the bus, he tells her that FU killed Roger O'Neill by putting rat poison in his cocaine, then says that he can't prove it because "all [of] Mattie's tapes were stolen". But the only copy of the tape where FU mentions the rat poison, was in the tape recorder that Mattie had on her when FU threw her off the roof garden. How would Krajewski know this?