The chronic pain example is that a prolonged duration (about 2 weeks) of being free of pain medications can cause the body to adapt and recognize it's over-interpreting pain.
The extreme phobia was if you had someone with a severe phobia of snakes, you would put them into a room with a few dozen snakes in cages and lock them in until they recalibrate as their body can only stay in panic for so long. However... it isn't the most ethical way as it actually is possible to suffer heart failure from fear.