What we tested
| File | UltiMaker Cura said | Actual problem | Result |
|---|---|---|---|
| 40 × 20 × 12 mm block with two triangles missing from the top | Model Errors | Small hole (4 open edges) | Same G-code as the intact block |
| Same block without its bottom face | Model Errors | 16 open edges | Same G-code as the intact block |
| Two 20 mm cubes merged, with the face between them kept | Model Errors | Internal wall (4 non-manifold edges) | Same G-code as the intact tower |
| Block with two triangles facing inwards | No message | 4 flipped-face edges | Same G-code as the intact block |
| Block exported with its hidden boolean cutter | No message | Cylinder inside the part (148 self-intersections) | Through hole printed solid; part 30 mm tall, not 12 |
| The block with the small hole after FixSTL repair | No message | Closed solid | Same G-code as the intact block |
What is going on
Cura checks the X-ray rendering of what is on screen, at most every 30 seconds and only when the view redraws (source); any pixel where the count of surfaces is odd shows the message (source). So it can appear late or only after you move the view, and it does not see faces pointing the wrong way or parts that overlap. "Learn more" opens UltiMaker's article, which calls the popup "Your model has missing or extraneous surfaces".
Cura does not change the file; its Mesh Fixes settings decide how the slicer treats the problem. FixSTL closed the small hole, after which Cura showed no message; it left the missing bottom face and the internal wall as they were.
What was actually wrong
How to fix it
- Open the X-ray view and look for the areas shown in red.
- Slice and check the layer view at those heights: small holes and internal walls often print normally.
- Check the file in FixSTL: it lists open edges, internal walls and overlapping parts, including the kinds Cura does not flag.
- For a large hole or a leftover cutter, fix the model in your modelling program and export again.
Source: Seen in our tests · UltiMaker Cura 5.13.0 on macOS