Flashbacks were something that I didn't believe in until a close & trustworthy friend told me about his. He was sitting in Denny's late at night as is his habit, reading and drinking coffee with whiskey from his flask mixed in, when the walls started to undulate. He sat back and let it pass. For him, it wasn't a negative experience, but a transitory curiosity.
Flashbacks are a re-occurrence of the effects of a hallucinogen, usually short and mild. They usually occur within a few days after a trip, and seem to be triggered by alcohol, marijuana, stress, caffeine, fatigue, or a history of mental illness. (Though as far as I recall my friend had not tripped recently when his flashback occurred, but it may have been related to the whiskey.) Most users (70%) never experience one, and for all the fuss that is made about it in anti-drug literature, flashbacks only have a 20-30% chance of occurring (Wikipedia).
Though science hasn't yet presented us with a definitive explanation for this phenomenon, it has been proven that they are not caused by residual chemicals trapped in the spinal cord. LSD dissolves in water very easily, and therefore gets washed out of your system within 24 hours (Shulgin). Edward M. Brecher has a theory, which makes sense to me, that flashbacks occur with any extremely emotional event (whether positive or negative) and so it is of no surprise that LSD use would result in them as well (XS4All).
It's possible that the key to flashbacks will also provide insight into the mystery of HPPD, or Hallucinogen Persisting Perception Disorder. I'll write a more in-depth post on that later, but for now, it's a syndrome that is defined by certain symptoms of a trip lasting a very, very long time (or permanently).
Many people find the threat of flashbacks a reason to avoid LSD, because of the unpredictability of the experience. However, in my opinion, if you're having a flashback you'll understand what's going on and be able to ride it out. If you were okay while tripping (which is likely) you'll most likely be okay during a flashback. If you have a history of mental illness, you shouldn't be doing acid in the first place.
It seems that although flashbacks do occur, they are a purely psychological experience, and don't pose any threat. Therefore, anti-drug literature that treats LSD as an insidious specter that will haunt you for the rest of your life is entirely incorrect. However, he may show up every once in a while to say "hi."