A mesh can render fine and still be broken in ways that surface later — at bake time, at export, at print, or at import into an engine.
Flipped normals
Every face has a front. A flipped face renders black, invisible, or inside-out depending on the renderer, and breaks lighting and baking. Most tools can recalculate normals outward automatically, which fixes the majority.
Non-manifold geometry
Geometry that could not exist as a real surface: an edge shared by three or more faces, a vertex joining two otherwise separate surfaces, a face with zero area. Renderers often tolerate it. Booleans, subdivision, baking and 3D printing do not.
Loose and duplicate geometry
Stray vertices, floating fragments, faces duplicated exactly on top of each other. Duplicates cause z-fighting — the flickering where two surfaces occupy the same space. Reconstruction produces both routinely.
Interior faces
Geometry inside the model that nobody can see. It costs memory, confuses ambient occlusion baking, and inflates triangle counts. Common in meshes built by combining primitives, and in reconstructions of objects with openings.
Open edges (holes)
Edges belonging to only one face. Fine for a plane or a flat card; a defect on anything meant to be solid. Fatal for 3D printing, which requires a watertight surface.
Overlapping UVs
Two islands occupying the same texture region. Deliberate for symmetrical texture reuse; a bug when baking, because two surfaces write to the same pixels.
N-gons and long triangles
Faces with many sides, or extremely elongated triangles. Both shade unpredictably and subdivide badly.
Order of operations
Fix normals first — many other diagnostics depend on consistent orientation. Then remove loose and duplicate geometry, then close holes, then address non-manifold edges, then look at topology quality. Reversing that order means re-doing work.