Moiré

The Moiré effect or, in other words, a halftone overlay, is a common phenomenon when viewing prints. It occurs when two even patterns overlap unevenly.

When does Moiré appear?

Moiré always occurs when screens overlap. Typical examples:

• You have scanned in a newspaper ad and print it in another newspaper.
• You print the portrait of a managing director wearing a jacket with a fine houndstooth pattern, a checkered shirt and a finely patterned tie. Regardless of the printing process, complete Moiré chaos is guaranteed to break out here.
• A brick building is reproduced in offset printing.
• The photograph of a ventilation grille is viewed on a monitor

You can find further information here:
What is Moiré? And can I see Moiré in a proof?