I was trying to create a hall of mirrors. A long hall with all the walls, floor and ceiling as mirrors. Some unexpected results in the render.
Create hall 2000m long, 5m wide and 5m high. Wall, floor, ceiling texture is Glossy Bsdf with roughness=0. Try also Principled Bsdf, no difference.
Place point lights inside every 25m, mid-width and mid-depth. 3000W and .25m radius. Camera setup at 1600m looking down the long hall.
In Viewport Shading see the hall as nice mirror look. Expect the Cycles render of the interior to be the same. Unexpected black/white result rendered given.