I hate when people act like know-it-alls. I hate when people hear some stupid tidbit on the "news" and take it to be some new fact of life. Some study shows that cell phones may possibly have a link to some type of cancer (or may not have a link) and people are all running around like "I knew it! Cell phones cause cancer!". I hate when people take some anecdotes as facts. A gathering of personal stories does not constitute evidence.
Scientific research is difficult. Don't assume you know better just because you're neighbor's third cousin had disease X that they just KNOW was caused by disease Y.
I study this stuff (study design, etc.) for a living. It's so hard to hear people shoot their mouths off about what they "know".
"I don't know! I don't know why I did it, I don't know why I enjoyed it, and I don't know why I'll do it again!" -BART SIMPSON