I've never had LSD flashbacks, but I've definitely had 2c-i flashbacks after I used too much, too often back in highschool. Really horrible ones and I'd get these awful migraines when they would happen. People would take on animal-like characteristics, that awful toxic pink and neon green scale pattern would be all over everything, I'd feel like being a person made no sense and was disgusting, and everything was awful. They finally went away permanently after I did a heroic dose of psilocybin. Haven't had one in a decade.
I've done a lot of mushrooms, a lot, and I've never had flaskbacks in that way. I'll remember or think over an amazing or powerful trip, but that's about it. Sometimes, if I wake up really fast in the middle of the night after a nightmare, I'll have mild visual hallucinations for a minute or two. That's about it. Don't even have HPPD anymore, just a little visual static that I don't even notice unless I really focus on it.