A missing or incorrect texture
Put the matching OBJ, MTL and one merged JPG together. Open the MTL in a plain-text editor and check that map_Kd points to the actual JPG filename, including case and extension. Confirm that the OBJ references the matching MTL. Reopen the complete set in MeshLab or Blender and check the color on the whole model before making the ZIP.
Too many faces
The source allowance is 500,000 faces per subject: two subjects would be 1,000,000 faces in total. More faces do not automatically create more physical detail. Reduce unnecessary geometry while preserving the silhouette and UV mapping. The file-printing request form is for one person or one pet; a multi-subject model needs an individual intake review.
Scale first, then inspect thickness
Check thickness at the final physical print size. The minimum wall thickness in the source is 2 mm. Long or broad features need additional thickening, and support joints cannot be too small or too few. There is no single source-specified diameter that makes every connector safe.
Count contact positions between subjects
The source first says at least three connection positions and later uses wording stronger than three. These are counts of separate contact positions, not millimeters. A connected multi-subject model therefore needs an engineering review of the contacts and their strength; do not replace the requirement with a 3 mm or 4 mm universal connector rule.
Remove nested geometry and internal shells
A model must not contain complex nested structures or redundant internal shells. Inspect a cross-section or reversed normals when hidden structure is suspected. Split connected components in MeshLab or another suitable editor, remove unused shells and check again. Component edits can break texture links or UV mapping, so reload the repaired model with its texture before submission.
Close holes and remove intersecting faces
The required result is a watertight solid with no unintended holes, overlapping or intersecting faces, or hollow interior. Use a mesh editor to locate holes and close them; resolve intersections while preserving the intended surface. The source gives MeshLab and ZBrush examples, but the acceptance condition is the repaired geometry rather than the name of the software.
A file upload is only the first check
This site checks ZIP structure, matching file types, texture resolution and face allowance at intake. Solidity, dimensions, thin sections, internal structure and multi-part connections still need human manufacturing review. A technically accepted archive is not a printability approval.