In "The Dumpling Paradox", the gang is playing Halo on Halo Night when Penny's friend Christy gets Howard's attention and he ditches them. Penny joins in and finds that she is a great Halo player. The next week, while Howard still has Christy's attention, the guys try to get Penny back to play Halo, but she is going dancing that night.
In "The Cooper-Nowitzki Theorem", Leonard comes home for Halo Night and Sheldon's self-appointed assistant Ramona Nowitzki will not let him participate in any of his leisure activities including Halo.
In "The Peanut Reaction", Howard is supposed to get Leonard out of the apartment, but he keeps playing Halo instead.