from what i remember[and its been a long time because LSD is almost impossible to find in NY city] the day after i had pains in my back and i remember when i cracked my back a few days later id experience a 30- 60 sec.flash back.
When you crack your neck you release small amounts of serotonin that can somewhat have an effect on an acid trip. very negligible and will NOT be permanent. Stupid urban legends. Almost as dumb as the Blue Star tattoo