This is by far my biggest pet peeve. You know, when people have such a strong opinion about one topic or another, but have taken absolutely no time at all to educate themselves on the said topic? Ignorance is not the same thing as stupidity. Some very intelligent people can also be very ignorant. They often are aware that there is other information out there, but often choose to ignore that fact because they have already framed their opinion and have no desire to read information that may contradict that opinion.
Most notably I have seen A LOT of ignorance concerning breastfeeding and nursing in public. People who have very harsh opinions on and judgments of when, how, and where a mother should breastfeed her baby. People who have perhaps never breastfed, never considered breastfeeding, can't breastfeed (men), and/or who have never read one single thing about the benefits of breastfeeding versus formula feeding. All they see is BOOBS. They turn a very natural and loving act between a mother and her child into something sexual and perverse. I cannot even tell you the utter exasperation and frustration people who think and believe those things causes me.
I am of the school of thinking that if I don't know what I am talking about, I either keep my opinions to myself or I choose to research and educate myself on the topic in question before opening my mouth (or my Blogger account or Twitter, etc.).