Why is the liquid's reflection look so weird after the render? (Blender Guru tutorial) I was doing the coffee cup tutorial that made by Blender Guru,and this happened:
The shadow looks weird when there is liquid in the cup.(Look like dimanods?)
But everything looks right when there is nothing inside the cup. Don't know why is that happening.
Here is my compositing.(I didn't turn on the denoising in the render properties.)
Here is how it looks like in rendered view mode

The wireframe of the glass and the liquid.
Cup's wireframe
LIquid's wireframe

Have a nice day.
It looks like your liquid mesh is intersecting very slightly with the cup.  It's possible they have different render values for their Subdivision Surface modifiers, or a common fix (and one given by Price) you could try is scaling the liquid mesh up very slightly.
This intersects the liquid's outer faces into the glass completely which gets you a pretty good render.
Additionally, there are several posts on this site and others regarding the proper physically correct method of rendering liquids in glasses, which I will link here.
Liquid / glass interface IOR and normals in Cycles - BSE
fluid in a glass - BSE
Fluid in a Glass - Greg Zaal's blog