The peekaboo bug is the name given to a bug in IE6/7 when DIV elements randomly disappear. Clicking refresh sometimes makes them reappear (hence "peekaboo"). The root cause is usually one of a variety of different CSS rules being applied to surrounding elements. One commonly used rule that can trigger the peekaboo bug is "overflow: hidden" applied to DIV elements, often used with the "clear: both" rule to expand DIV containers around floating child DIV elements.
The workaround I have found most useful is to put a "zoom: 1" style on the affected DIV. This rule causes IE to layout the element thus forcing it to appear.