Visual artifacts, lessers to those acutely experienced during peak intoxication, may simply be normal (not previously noticed) visual phenomena that one falsely attributes to the drug; others are rarely explained as natural (non-psychedelic) phenomena. These diminish, perhaps over the course of several years; likewise, non-visual sensations can be associated with tripping, long afterward.
I would say this is fairly common and may become worrisome reminders (neurosis) following a bad trip.
Other things not really explained can occur afterwards, such as brain zaps, HPPD, and prolonged psychosis.
HPPD is when visual acuity is disrupted to a degree causing a disability; these are described as visual snow or intrusions of color in the field of vision. The risk of HPPD does not seem dose-dependent. Flashbacks have been subjugated as HPPD.
The risk of real psychosis, where you get delusions or experience hallucinations, increases from mixing drugs, e.g. marijuana, or taking higher psychedelic doses.
Brain zaps, to my estimation, are like confused electrical impulses in the brain that produce a mild seizure.