Why can a 3D model fail during STL export even when it looks correct?

[indiancad]

A 3D model can look perfectly fine on screen, but problems may appear when exporting it for 3D printing. Open edges, overlapping faces, internal geometry, or non-manifold areas can make a mesh difficult for the next application to process.

The tricky part is that these issues are not always obvious from the model's appearance.

When this happens, I would rather identify the underlying geometry problem than keep repeating the export.

What checks do you perform before exporting a 3D model to STL? Do you validate the solid in CAD first, or use a separate mesh-repair tool?