A lot of the self-published books on Kindle have clearly never seen the eye of an editor - my dad loves the free or dirt-cheap ones and then moans about spelling/grammar errors...
Happens because the easiest person to fool is yourself; not just things like confirmation bias or inaccurate memories of an event - your actual sensory input itself can be unreliable whether it’s visual tricks like patterns that seem to move when you focus on them, the brain filling in words you forgot to actually type in a sentence but read aloud in your head, or phenomena like inattentional blindness (see the famous invisible dancing gorilla experiment). Only human!