Panic selling on market causes prices to drop quickly causing more panic selling.

Weight gain that causes negative emotions that causes excessive eating that causes more weight gain.

In an economic recession, job loss leads to decreased spending, which harms businesses and leads to even more job loss.

High oil prices lead to high gas prices, which makes people drive less so they don’t have to pay a lot for gas. This leads to less demand for oil, stabilized prices, ultimately leading to more people driving again.

A new social media site starts up and gains a few followers. Those followers share the site with their friends, who also become followers and share the site with more friends. Eventually the site has many followers.

When the population of deer in an area begin to grow, they start overgrazing the vegetation that feeds them. When there is not enough vegetation to sustain the rapidly growing deer population, they will begin to die off until the vegetation grows back to normal levels and the deer will have enough food to repopulate again. 

Earth warms, so ice begins to melt. As the amount of highly reflective (high albedo) ice melts, the earth warms faster.

At the edges of deserts, reduction in vegetative cover due to low rainfall may lead to reduced precipitation. This leads to expansion of deserts.