A simple confounding case: in this model, given Z, there is no association between X and Y. However, not observing Z will create spurious association between X and Y. Z is called a confounding factor.
Waiting time between eruptions and the duration of the eruption for the Old Faithful Geyser in Yellowstone National Park, Wyoming, USA. This chart suggests there are generally two 'types' of eruptions: short-wait-short-duration, and long-wait-long-duration.
The placebo effect can be produced by inert tablets, by sham surgery, and by false information, such as when electrical stimulation is turned 'off' in those with Parkinson's disease implanted brain electrodes.
A funnel plot expected without the file drawer problem. The largest studies converge at the tip while smaller studies show more or less symmetrical scatter at the base.
A funnel plot expected with the file drawer problem. The largest studies still cluster around the tip, but the bias against publishing negative studies has caused the smaller studies as a whole to have an unjustifiably favorable result to the hypothesis.