Moiré Patterns

When a transparent pattern lays over a second pattern, the combined patterns are called a Moiré [more-ray].

The word Moiré was first used by weavers and comes from the word mohair, a kind of cloth made from the fine hair of an Angora goat.

 A moiré is a convergence of different patterns. When two patterns combine they create a pattern of their own that does not exist in either of the originals. This interference between two patterns is called a Moiré pattern.  This can happen, for example, when scanning large areas of what seems to be a single color. Sometimes what we perceive as a block of color, is actually a great many fine dots of ink. When printed, small random variations in value can occur due to the imperfections of print technology. The grid pattern of the scanning process combined with these almost-invisible random ink patterns can produce unexpected results. This is often in the form of a mysterious "banding" or off-color   "stains". Scanning an image slightly off "square" can increase the likelihood of this happening - the original printed "grid" of dots in the original is out of line with the scanning grid. 

If the scanning software offers "de-screening" controls this can often also solve the problem of  moiré patterns.  If it's still a perceptible problem, a small amount of Gaussian blur can help in some situations. But also remember to try a higher scanning resolution.