I'm practising with a mesh object typical cube. Added a whole bunch of cuts in the mesh. I believed the mesh is clean with no problems, until I added the subdivision surface, and down the corner of IMG 03, suddenly it's creating a triangle and also some weird unclean artefact on the surface. That I sort of cleaned adding more loop cuts, but it didnt clean 100%. But that corner triangle I cant figure out what it can cause it.
Your mesh has some n-gons and the Subdivision Modifier doesn't like it. :)
Make your mesh only from quads and it will subdivide properly.
Other little issues:
- Messed up scale - Solve through Object > Apply > Scale (and check normals afterwards)
- Non-manifold faces - some faces are going through another. This has to be repaired manually. Either merge the vertices, or move the faces farther away.
Quads vs. N-gons with Subdivision Modifier
Apply your object's scale to avoid problems in the future.