Lay 12-pixel lines over 13-pixel lines and a slow pattern 156 pixels wide appears from nowhere. Guess the width of the ghost stripes before the maths tells you, then break the rules in the lab.
Each grating is a wave with a spatial frequency, 1 divided by its spacing. Where two waves overlap, the picture contains their difference as well. For parallel lines with spacings a and b the ghost stripes repeat every a x b / |a - b|. That is why 12 and 13 give 156 while 12 and 14 give 84. Close spacings make huge stripes.
With the same spacing but a rotation of angle t, the ghost width is a / (2 sin(t/2)). A tiny turn of the blue layer gives wide stripes that sweep fast. In the lab the readout uses the general version, a x b / sqrt(a^2 + b^2 - 2ab cos t), for lines and dot grids.