Once Rainbow Dash used going to the bathroom as an excuse to find a last-minute present for Fluttershy. But on a separate trip to the mall, she did have to go to the bathroom really badly. By the time she got to a bathroom that wasn't full, she wet her pants. She sat on the toilet to avoid making a puddle, then rushed back to the others in her wet pants. This being far from the first time she wet herself (including multiple deliberate snowpants wettings when she didn't want to pause her winter activities), she's barely embarrassed and will laugh about it.