Had a root canal on an eye tooth years ago after the crown had broken off. Trick was...the root was still infected. The dentist froze around it 4-5 times but the freezing never took. So he decided (without warning me beforehand) to put the freezing needle directly in to the nerve in the middle of the root.
The jolt of pain was like a white lightning bolt and damn near tore my head off. It did the trick and froze everything within seconds - but that's one pain experience I'll never forget - and I generally have an extremely high pain tolerance.
Never let a dentist pull a stunt like that
Having 29 extractions in one afternoon years later didn't come close to equaling that pain, lol.