Images flash before your eyes. The slower they flash, the more likely they are to cause headaches. In motion pictures, old movies flashed at a slower rate, hence the choppy movements.
A more serious problem is using a monitor in a room with flourecent lights. These flash at 60Hz in North America 50Hz in most other countries. If your monitor is flashing at 60Hz, it can cause the image to look choppy. If it fluctuates just slightly from the lights, it can make scrolling bars appear. Not a good choice.
Now, problems could occur at even multiples of 60Hz, such as 120Hz. Or even at close fractional multiples, such as 90Hz.
It's my opinion the smoothest (least flashy) image comes in frequencies between 70 and 85Hz, or 100-110Hz (at least on the North American power standard). Since most monitors can do 70-85Hz, they're good choices.
Moire affects the shape of the picture at the sides, use it to adjust your monitor so the image fits squarely on the screen.
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