My mesh has this strange shading:
I double checked there is no overlapped vertices. Also the edge flow seems normal:
It happens even after I triangulate the faces:
What’s happening here?
Hi @raincole, check if you have doubled verts, faces, or flipped normals, these are the most common issues. Go to Edit mode, vertex view, select all vertices, and search for “Merge by distance” (for me the space bar is the search, which I strongly recommend to keep so).
So it should take care of doubled faces and vertices. You can also check for normals, in Edit mode, everything selected, and Alt + N, Recalculate normals (there are inside abd outside options, you will see which is yours). Alternatively, you can set the face orientation view here, and manually check for faces you need to flip. I intentionally flipped one face normal to look inwards, so I can show you:
So either one of these should solve the issue 
Thank you very much for the reply, but I tried both Merge by Distance and Recalculate Normals and neither fixed it.
The face orientation looks like:
The best way to get an answer is to upload the geometry in question. There are too many possibilities.
Check for doubled or internal faces: that bites me on the regular. Using ALT+B to hide geometry makes this easier.
Ok, I uploaded here:
Thank you! Indeed, Merge by distance might move vertices out of their desired place. I assumed your faces might lay precisely on each other, but there were some errors, since one of the vertices moved upwards. Please go back to the original position where the cylinder is precisely set, and check for the errors there. In this file, the shading is wrong because the faces in question are not flat, they are twisted, that is why the issue happens, at least here. Please Ctrl - Z back and check if the faces are flat around that area, or maybe they are twisted for some reason. That can cause this issue for sure.
One answer is that you have Custom Split Normals on your mesh (whatever they are).
If you go to Properties/Object Properties/Geometry Data and hit “Clear Custom Split Normals”, this will remove the artifacts in your screen grab. If you are using them, well, then you must know what they are.
These get added automagically in various operations. I’m not sure of their function.
Always upload a sample for quick answers.
Wow thank you very much!! I’ll never find the problem by myself. It’s modified from a downloaded model.
Even on hand-built models the custom-split-normals thing can appear. IIRC it’s usually when a modifier is applied.
There’s a hierarchy of things to check when you have render anomalies, this is probably #4 or so.
Please mark my comment as “Solution”, so I get forum brownie-points.