Novelties such as?
Papal infallibilty and the Immaculate Conception, for starters.
Then, there’s that little historical matter of the Bible and mass in the vernacular, communion in both bread and wine, etc. Descendants of those originally proselytised by Cyril and Methodius were greatly offended when these were taken away from them by the Roman church, and that offense continued on for quite some time with their descendants in turn, leading to an outright break with Rome and the usual medieval response.
You know, that whole Jan Hus situation?
Or, maybe you really don’t know.