When I was young I thought the concept of an "acid flashback" was at the very least a misleading way to describe things like residual hallucinations or other relatively negligible side effects, I could deal with those. But having a proper "flashback" out of my control definitely taught me otherwise.
I took 6-8ish LSD trips when I was 16-17, I don't remember ever taking any more than two tabs though, not a ton of cumulative exposure but I suppose I was young with a developing brain. I was smoking a bowl one day with the dudebros while picking up weed and it wasn't until well after we finished smoking that I felt it happen: I was sinking, sinking deep. The big screen TV rose higher and higher along with the walls, and I felt like it would continue until I couldn't look any further up. I felt like I was tripping too, body high like the first "oh I'm starting to FEEL it" moment I knew and loved. It only lasted a few seconds and I was pretty startled having felt it so strongly and so suddenly.
A few months or so after that I was smoking riverside when I looked at a lone flower nearby and felt it happen again. No intense perception games this time, but the body high was unmistakable, albeit not as strong as it was last time. Only happened for a few seconds again.
I stopped taking psychedelics for quite a number of years after that, even though it seemed to be very much tied to smoking/other psychoactive substance use, it just made me feel uncomfortable to know that I could "flashback" while driving or doing something similarly dangerous.
Pretty much everybody I've ever talked to about their HPPD has been just about residual visuals and I feel kind of pissed off that I ended up with that bullshit, it was a big part of the reason why I avoided taking psychs again for so long, at least LSD in particular.
Considering I have a relatively clean slate of a body chemistry to toy with these days, I've been thinking about continuing to stay away from LSD while I keep taking other psychedelics. But goddamn if I haven't been aching for my childhood love.