It is not surprising that a large perturbation to a system can result in sudden transition from one state to another. For example, you are sleeping and someone comes and shout your name, sudden transition from state of sleep to wakefulness.

Mysterious and most interesting kind of transition is when gradual small perturbation to system can result in sudden transition from one state to another. A clear lake can shift to turbid lake within few days for the gradual small increase in nutrient load in lake. Upto a threshold of nutrient, a lake remain clear but the same small increase in threshold, lake will shift to turbid lake. And its not easy to achieve the state of clear lake by reducing the same amount of nutrient.
Ecological system such as African savanna sudden shift from grassland to tropical forest for gradual increment in rain or other factors.

In some transitions, it is not possible to achieve a previous state by taking some step back. The system requires a very large cost to achieve its previous state.

For example, play this game, incline yourself as much as you can, a threshold point comes when the body loses its balance and fall down. If you want to achieve the same inclination that was before threshold point. You have to stand again and repeat the process.

The only way to prevent such transitions is by taking precautionary steps at a sufficient pace before the threshold point.