Surely someone else has noticed this. In the cave tunnel between pirates spawn and the bridge to knights, there is a little area where the bricks like to make a splashing animation. I'm not entirely sure if it happens when people walk there, or swing their weapons, or something else maybe, which has made it tough to get a screenshot. Happens rarely enough that I forget about it but often enough that I always think "there is that glitch again."
I have yet to get a proper screenshot.