Trigger warning: Quotes of things that shouldn't be said. They can be ableist and triggering.
There could really be a hundred or a thousand of these, but I've decided to choose just fifteen for the sake of brevity and not imploding anyone's browser. All of these things have actually been said to Autistics, children and adults, and some of them are unfortunately very common. Some happen more often over the internet, and some happen more often in person, but they're all phrases or questions that can be incredibly hurtful. Sometimes people who say these things are well-meaning, which can make the impact even worse. Especially in those cases, people might not understand why these can be so offensive and hurtful, and occasionally insist that what they're saying is a compliment, even when it's not.
1. "So is that like being retarded?"
Factually speaking, Autistic people in many cases do not have an intellectual or cognitive disability, and many people with intellectual or cognitive disabilities are not also Autistic. There are some Autistic people who also have an intellectual or cognitive disability. Nevertheless, the word "retarded" is often very hurtful for Autistic people, as it is frequently used as an insult to dehumanize people with developmental and intellectual disabilities. The r-word is often used to express hatred for people with disabilities. Please don't use it.
2. "You should be very proud of yourself. You seem so normal. I couldn't tell that you're Autistic."
While this is rarely said to Autistic people whose disability is very visible, it is very frequently said to Autistic people with much more invisible disability. It's insulting because it suggests that because the person doesn't appear to be disabled or doesn't fit preconceptions of what Autistic people are supposed to sound or act like, that person must therefore not have a disability or be Autistic. It also suggests that "normal" is the standard to which anyone should aspire to appear or act (and that "normalization" should be the ultimate goal of therapies or treatments for autism rather than pragmatic coping skills to navigate a world where Autistics are a minority), and therefore that it's not good to act or speak in ways commonly associated with being Autistic, even if those behaviors don't actually hurt anyone. This is very dismissive of a person's disability and experiences.
3. "You must be very high-functioning."
Many Autistic adults take issue with the "high-functioning" and "low-functioning" labels for a variety of reasons. Some people have received both labels but at different times in their lives, and many Autistics have very uneven skill levels -- some people who might be able to articulate their ideas very well at a conference may be unable to travel alone or cook for themselves, while some people who are unable to communicate with oral speech might be able to live independently. That debate aside, this is also very dismissive of a person's individual experiences with disability. Unless you know someone very, very well, you have no way of knowing what specific adaptive functioning skills or life skills a person has or what his or her needs and challenges might be, and it's not possible to acquire that information simply by looking at a person.
I joined Georgetown University's disability awareness club, DiversAbility, now in its second year, upon arriving at campus. During one of our previous meetings, one of the club's officers mentioned that we will be hosting an "Ability Lunch," which had been done last year, in which people sit at different tables and simulate different disabilities -- for example, wearing a blindfold to simulate blindness or having one’s arm tied behind one’s back to simulate inability to use a limb or lack of a limb -- while eating lunch. I immediately raised objection to the idea, and was told that the discussion following the lunch included criticism of the event. 
