At one point during the slurping noises training, Kanna starts absentmindedly sucking her finger in a rather suggestive manner... in the school classroom, in front of her friends. Made even funnier in animated form in the OVA by cutting out the establishing shots of Hadzuki and Kotori.
Fumika and Kanna's seiyuu contest to see who can get Motoki excited using only their voices. Fumika goes for the slutty approach and is shot down, while Kanna launches an all out moe attack. It'ssupereffective!
And then it happens again during Hadzuki's and Kotori's training, only they work together for added impact.
In chapter 24, Kanna and Motoki are out in a hallway, having a moment, which gets completely ruined when Motoki turns around to find virtually the entire remaining cast peeking at them from across the stairway.
Chapter 25. "Go ahead and be forceful, I say! Kanna-chan's a pushover!"