If you have ever dragged an STL into a slicer and got an error, or a model that looks fine on screen but prints with gaps, the problem is usually the mesh itself, not your printer.
Here are the most common causes.
Holes. A hole is an opening in the surface where an edge belongs to only one triangle. Slicers need a closed, watertight shell so they know what is inside and what is outside. A tiny gap from a rough export can be enough to break the whole slice.
Non-manifold edges. This is an edge shared by more than two faces, or two bodies that touch at a single point. It often happens when parts were joined without being unioned properly. The slicer cannot tell which side is solid, so it guesses or gives up.
Flipped normals. Every triangle has a direction. If some faces point inward, the slicer may treat solid areas as empty, or fill the wrong side. You will often notice missing sections or dark patches in the preview.
Degenerate faces. These are triangles with zero area, or vertices sitting on top of each other. They are invisible in most viewers, but they confuse the math that decides how each layer crosses the surface.
You do not always need to reopen your CAD program. For quick fixes, a browser-based tool can check the mesh, point out these problems, and clean up many of them in a few clicks. A tool like STL Repair is handy for small jobs such as closing a hole or recalculating normals without a round trip through your modeling software.
A practical order of operations: run a check first so you know what is wrong. Fix normals and degenerate faces, close any holes, then export again. Compare the repaired file against the original before you print, especially for parts with tight fits.
If the model still fails after that, the problem is usually upstream. Re-export from your CAD tool with a finer tolerance, and check whether the object was built from overlapping solids that were never combined.
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