Moiré

Moire is what you get when two regular fine patterns overlap slightly out of step, either rotated a few degrees or at slightly different pitch. The mismatch beats against itself and your eye reads a third, much larger pattern that exists in neither original: wavy bands, shimmering checkers, rainbow ripples.

In print it shows up three ways. Scanning artwork that was already halftoned screens it a second time, and the two dot grids fight. Plate screen angles set too close together wreck the rosette. You cannot give four screens 30 degrees of separation each inside the 90 degrees a dot grid actually has, so the standard compromise spaces the three loudest inks (cyan, magenta, black) about 30 degrees apart and parks yellow in the leftover slot 15 degrees off a neighbor, where it is pale enough not to shout. And fine art detail (pinstripes, hatching, tight architectural lines) can beat against the screen ruling itself. Camera sensors do the same thing to fabric, which is why suit jackets shimmer on video.

Fixes, roughly in order: get the original digital art, use the scanner's descreen setting, blur slightly and re-sharpen, rotate the original about 15 degrees on the glass, or switch to stochastic (FM) screening which has no fixed angle to clash with. The trap: your monitor invents its own moire when it downsamples a high-resolution file, so a pattern that panics you at 50% zoom often vanishes at 100% and never reaches the paper. Check at full size before you redraw anything.