I was contected with a problem about imported meshes not exporting correctly, even if they hadn't been modified. After a little investigation it turns out the exporters can't handle a specific scenario correctly and no one noticed for 20 years. Or people have noticed and not done anything about it.

If you're trying to import a mesh with a skeleton and a bone is named the same as a mesh object, the mesh object overwrites the bone. This means there are bones missing. Which cause export errors and a bit of confusion. The fix was so simple even I could do it. I did do it.