Checkershadow illusion:
The square marked A is in fact exactly the same shade of grey as the square marked B! (If you can't see it, try covering up the checkerboard to either side of A and B.)
How it works: To make sense of our environment, the brain needs to work out the colour of objects. Shade tends to dim colours, so our visual system compensates for this.
In this image, we apparently “see” a three-dimensional cylinder casting a soft -edged shadow over the checkerboard. So the brain assumes that square B is a lighter square that is dimmed by shade. In fact, it’s not lighter at all.
The illusion is aided by the checkerboard pattern. The brain notices how an object contrasts with the things around it. If a square is lighter than the surrounding squares, the brain reads it as an unusually light-coloured object; the same for a dark square surrounded by light ones.
Why it is important to paint carefully when using multiple cans of paint for a roof, walls, floor, etc. Always finish in a corner, at an edge, in a natural break in the surface, in a straight line, then open a new can and start the new area.
If unavoidable and practical paint a dark line or stripe between the two colours. That way if the two cans or packs of paint are slightly different in colour they will still look the same.