Vaccines. A substantial proportion of people refuse to accept the reality that vaccines do not cause autism. This was never controversial in the scientific community. After the fraud Andrew Wakefield published his sham "study" linking vaccines to autism, the New England Journal of Medicine published the results of an absolutely enormous study (including more than 537,000 children) that thoroughly refuted Wakefield. Since then, the evidence has grown even stronger, concluding once and for all that vaccines are not a cause of autism.
Child Car Safety Seats. At least one lunatic believes that strapping a child into a car safety seat increases the risk of autism. How so? Chemicals. To prove his case, he shows that as the usage of car safety seats has increased over the years, so have cases of autism. QED. There's only one problem: A lot of things have increased over the years, such as sales of organic food. Possibly organic carrots cause autism, too?
Parenting style. Some people make great parents, while others stink at it. But parenting style itself won't cause autism, as was once elaborated by the "Refrigerator Mom" hypothesis. Instead, parenting style can ameliorate or exacerbate the symptoms of autism.