In the last episode of Code Geass R2, after Lelouch died, Nunally touched his hand and she suddenly knew what he did and what the Zero Requiem was. Was this a power of Geass?
This has something to do with the Geass. In episode 11 of Code Geass, Lelouch saw C.C.'s memories when he touched her. Though it wasn't the first time that he had touched her, seeing her memories was a first. It is left unclear under what circumstances does a memory leak like this one happens.
What happened to Nunnally was something similar. She saw his memories when she touched him. It is probably some anomaly related to the Geass.