The sliced preview does not match the model.

What it means

Your model looks fine because your CAD program renders surfaces. A slicer has to decide, at every layer, which side of every line is solid. A mesh can look perfect and still leave that question unanswerable.

How to tell which it is

This is the part worth getting right, because it decides whether you fix the file or change slicer.

The mesh	The same fault appears in a second slicer, and a checker reports open or non-manifold edges. Fix the file.
The slicer	The mesh is clean, and another slicer produces a correct preview from the same file. Report it, and slice it elsewhere meanwhile.

What causes the mesh version

• Open edges — the slicer has to guess where the inside is, and guesses differently on different layers. Missing walls are the classic sign.
• Self-intersections — two parts of the same surface pass through each other, so a point is both inside and outside. Cavities fill in.
• Inverted faces — solid and empty swap over, which is what produces a shell where you expected a solid.
• Duplicate faces — two triangles in exactly the same place cancel each other out.

Three ways to fix it

1. Slice it in something else to establish which of the two you are dealing with. Five minutes, and it saves fixing the wrong thing.
2. Repair the mesh if the fault follows the file. Close the open edges first — most of the other symptoms follow from those.
3. Send it to us. Self-intersections are the hardest class to repair automatically; we will do it properly from £15, or redraw from £45.

Settle it in a few seconds

Run the file through the checker before you reinstall anything. If it reports open edges, flipped faces or loose shells, the mesh is the problem and no slicer was ever going to read it reliably. A clean result points at the slicer instead.

Other slicer errors

• Bambu Studio: Non-Manifold Edges
• No Layers Were Detected
• Cura: Model Is Not Watertight
• PrusaSlicer: Model Errors and Auto-Repair