That it's a temporary illness. (I've also had people - including health care professionals! - make this assumption about my general anxiety disorder. Like WTF?! No. These are chronic ongoing conditions.)

That it can be completely cured.

That because I've never been in combat, or violently assaulted, or in some sort of horrific accident, that I don't actually have PTSD. Wrong. Mine is known as Complex PTSD. It's the cumulative effect of many "smaller" traumatic incidents that have occurred at various points in my life from birth until recently. The effect is the same.