A lot of people seem to immediately equate one having religious or spiritual beliefs to being an ignorant, hypocritical Christian fundie or a war mongering Muslim.
A lot of people seem to think Atheists are all anti-theists and immoral, nihilistic hedonists. They also incorrectly throw the word "belief" around when referring to atheists.
A lot of people don't understand that agnosticism doesn't necessarily exist independently of other religious (or non-religious) views.
In terms of medical stuff: people seem to think that having quick "mood swings" suddenly qualifies you as bipolar. No.
Also, no one even seems to know what Ehlers-Danlos syndrome is.
They also don't understand the more severe medical effects of joint hyper-mobility. Not just regular laymen, doctors too. ORTHOPEDIC doctors. There's very little more frustrating than going to the doctor and them saying "oh, a bit of extra flexibility is no big deal", despite the god awful pain perpetually ravaging your joints.