Kitty had known she could phase but never told anyone. Professor X used Cerebro to discover she could do so. When in the Xavier Mansion she phased through the Danger Room controls when Nightcrawler scared her. She phased again when Magneto and Juggernaut were attacking. When the X-Men were attacking the Brotherhood of Mutant Terrorists aboard Asteroid M Kitty used her phasing technique to get to the control room and help Nightcrawler when Magneto attacked him.