I am one of the people who can experience painful withdrawal effects: upon significant disturbance of coffee intake amount(less)/time after a few days at 3/4 espressos a day, I would experience a painfull and incapacitating headache in the following 12 to 24 hours.

I tested it, yep, was coffee (did not test with another source of caffeine though). Had 100% "success" over a few tests.

It is vicious: I don't need to "stop" drinking coffee to experience the headache, just need to disturb significantly the amount(less)/time of intake.