LSD bonds with something in the brain, such that it causes persistent flashbacks more than any other hallucinogen, sometimes years later. Taking it just once to try it might not have any effect, or might cause flashbacks for the rest of your life. Depends on your personal neurochemistry, the dosage, form of dosing, and your mental state. A friend who had taken LSD described it to me once something like this: LSD sometimes takes any cracks (neurosis or psychosis) in your mind and expands them wide open, forcing you to look into them. If you're aware of them already and they're minor, it can be helpful, sometimes expanding your mind to new capabilities. If you've been unaware or worse, in denial, or they're major, then you get a bad trip that may come back to haunt you over and over again over the years.
Flashbacks that continue to occur after the original drug effects have worn off are a medically recognized phenomenon, which is documented in the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) as hallucinogen persisting perception disorder.
Flashbacks typically happen in the days or weeks following ingestion of the drug but can happen months or even years after the drug use has been discontinued.