I was trying to create a sideways piston extender (or whatever it's called) underwater which had soul sand underneath so that it would create bubbles every time it opened, but then the bubbles somehow turned temporarily into this weird rainbow block and pushed the walls of the contraption out, causing it to get flooded. I managed to take a screenshot at the exact moment it appeared, it happens every time the piston pushes the other piston outwards. I also took a screenshot of the whole contraption itself, without the water flowing through it.