Mio was carrying Yuuko's wallet in her bag for some reason (why?). Then the moment comes and Mio realizes that the wallet is not there. They go to the lost and found department. They found the wallet and we see inside of it as in the screen shot below. Yuuko's day is ruined after see that paper and the green thing inside.
What was that paper and the green substance that put Yuuko in such a bad mood?
Inside the wallet:
Yuuko's mood for the entire day after this event:
(Location: Episode 12; at the beginning of the second half.)