What happened to the punishment for stealing an S-Rank Quest?
Episode 19 (filler episode) and episode 20 (omake episode) are non-canonical from a continuity perspective. In the manga, when they return to Magnolia, Erza reminds them about that punishment (just like in the anime), but then they see their guild building destroyed. This begins the next Phantom Lord arc, and thus the punishment matter gets dropped naturally. Since the punishment was never shown in canon, they probably decided to skip it in the anime too. 


   In any case, that punishment is most likely something silly, such as the one in episode 75 later.