As always, the devil's in the details. You will note, that like the Viet Vet
Dis Myth, the issue is not whether it ever happened, but whether it is a significant causal relationship, and if what we remember is what actually happened. Even if the effect existed, it certainly was not as ubiquitous as its proponents claim. Taking lysergic acid can be a profound emotional and psychological experience, like combat, prison, rape, or even an auto accident. We are now doing intensive studies on the effects of post-traumatic stress disorder, whose symptomology is very reminiscent of the alleged flashback phenomenon. Perhaps those documented cases of flashback are more about PTSD than LSD.

There are also thoroughly documented effects of a strong placebo effect here, I have personally witnessed people getting stoned after smoking oregano and catnip, and remember I once had a mild flashback-type experience immediately after seeing the film "The Trip", with Peter Fonda and Jack Nicholson, a full two years before actually trying LSD myself! As we used to say in the those days, "Did you really have a hallucination, or do you just think you did?"

The political and cultural baggage associated with drugs, especially those associated with the radical '60s youth movement, makes it very difficult to accept uncritically opinions from any agency associated with the Federal government on these topics. The government war against marijuana, long accepted by society as an innocuous and benign weed, continues even though it has been thoroughly discredited.

It was often remarked during the Viet Nam era that even while full scale war was being waged on the popular psychedelics, the pharmaceutical industry was developing and marketing a powerful suite of psychoactive stimulants and depressants, prescription substances with limited medical use and great potential for addiction and harm, while the authorities were making no effort to introduce special legislation or action to control their use or their propagation in the illicit drug trade. Some cynics even suggested it was no accident, John Mitchell was Attorney General.

There is always another side to the story, and research to support it, if you just look long enough.

http://www.maps.org/research/abrahart.html