At the mall, Rainbow Dash had to go to the bathroom really badly and ran all over the mall searching for a bathroom, but all the stalls in every bathroom were full. After revisiting them multiple times, she ended up wetting her pants.

Of course Fluttershy doesn't think any less of her, being prone to pee accidents herself.

(Yes I know in the actual episode she didn't really have to go to the bathroom, she was just looking for a last-minute present for Fluttershy, but it's still a fun thing to think about.)