Uncommonly, doing the jumping Cerulean Splash causes Miriam to stick her hand out but the spell won't come out.
It can take a while to happen so it's easiest to reproduce in a save room where you can recover your MP. Just keep jumping and casting. On average, I think it happened once per two full magic bars, so it's not frequent. But when it happens in the heat of battle, it's annoying.
Don't know if it's related, but spamming Cerulean Splashes to recreate the bug made the game laggy. Is the spell effect CPU intensive?