Rendered image has white "tiles" Still very new to Blender. Trying a first mini-project after following multiple tutorials.

This is a plane with a seamless sand image placed on it (through shown nodes), with an as-yet untouched cube. A plane provides lighting distant far up and behind (per given view) the block.

In render mode, as seen in small bottom area, things look okay. But the top is the result of rendering the image, lots of white squares and triangles, which are patterned to the subdivisions on the plane.

The surface has not been height-distorted (all 0 on Z-axis), it's just a simple large plane that's had several subdivides as shown here:



I've not seen such things during learning with tutorials, and while I could find lots of responses to white "fireflies", I couldn't find anything on what could cause this rendering issue.

Where should I be looking?
I think you might have a duplicate ground plane in your scene that is hidden in your viewport (which is why it does not show up in the viewport rendering). If that is the case, it will still be rendered because the F12-rendering does not ignore hidden objects.

If the plane has a white material and is in exactly the same z position, it will be in conflict with the textured one. As a result, faces will show pretty much randomly from either of the two objects, creating the pattern that you can see in your rendering.

You can unhide all hidden objects with Alt+H in the 3D View.