Gotchas

Mistakes real passes made, each with its fix. When a pass hits a new one specific to the project, it goes in the style guide’s Pitfalls.

Geometry

Coplanar by construction. The usual fights on built things: two boxes’ bottoms both resting on one floor (sink the plinth, stand the walls on it), crossed bars in one plane (part them in depth), rails or boards meeting at a corner (fit two between the other two), a hole cut flush with its frame’s edge. A first audit of a hand-built model often lists hundreds of pairs; most are these four.
Lathe winding. A lathe profile faces outward read bottom to top. A cap or lens wound from the centre out faces inward and vanishes from outside. A disc facing up goes rim → centre.
Creases through helpers. A shape helper that takes a crease angle must pass it all the way down. A helper that drops it falls back to the default, and segments wider than that angle (10 segments round is 36°) shade as flat staves.
Tubes cut corners. A Catmull-Rom tube through a few points cuts the corners between them, straight through whatever it wraps. Build a chain or rope that must hug a surface from that surface’s own geometry.
Cheaper-looking can cost more. A lathed mound can carry more triangles than the primitive it replaced, and × hundreds of instances that matters. Measure after every swap; revert when it costs more.

Fixed-size shader features. A shader detail with a fixed world size (a wood knot, a stain) can read as a hole or a blot on a large surface. It shows only on other objects that share the material, so after a shader change, screenshot every studio area that uses it.
NaN and black frames. A NaN position or zero-length normal lights a pixel NaN; bloom spreads it into a black block. audit() finds the geometry cause. Extruded outlines on moving parts have caused black frames with no NaN found: prefer primitives for moving parts, and run the black-frame check (blackFrames()) after adding new geometry kinds to anything that moves or glows.

Verify what the merge skips. A bake/merge step meant to skip some objects (text, animated parts) often decides by a marker flag, and libraries change their flags: one project’s hand-rolled merge swallowed its troika text for weeks, and the signs went blank. three-batch’s bake skips troika text, instanced, skinned and morphing meshes; whatever does the merging, check the merged object in the running app, not only in the studio.
One mesh over the world. A static InstancedMesh or merged geometry spanning the world has one bounding sphere, so it is never frustum-culled: every instance draws every frame, in the shadow pass too. Split it into cells (chunkInstances / chunkMesh from @zkmake/three-batch). One project’s frame went from 608k to 169k triangles.
Count before calling it cheap. A rare-looking piece can be numerous (86 stumps a level). Count from the placement code, then multiply.

Look at the shot before judging it. Views have missed the object (a wrong coordinate formula), started inside it (camera within a rock), or looked end-on. Every new view gets screenshotted and looked at before any numbers are taken from it.
Scatter areas need a seed with the pieces. A world-slice area generated from a seed that lacks the rare piece measures nothing. Pick or force a seed that contains it.