A Little Knowledge Is A Dangerous Thing

Paraphrase of two recent comments I've seen:

"The UK National Blood Service are asking for donations of O, A, and AB [but not B]. Ha ha, if they'd paid attention in high school biology they'd know that ABs are universal recipients, so they can use B just as well as AB."

"Census has a free-text field for listing your occupation. Anybody who's done a first-year survey design course knows that free-text questions are useless for quantitative analysis."

And so on. People who work in $FIELD for a living do something that looks stupid to a layman with even the smallest shred of knowledge about how $FIELD works, and mockery ensues.

Now, I'm the last person to say that you should assume competence; people who should know better quite often DO get stuff wrong in their own field. But it's also a bad idea to assume incompetence. Very often, including both the examples above, the commenter's limited understanding of $FIELD is wrong and the "experts" really do know their job.

So, usually a good idea to check the facts quietly first before going all Nelson Muntz on somebody who's quite likely to be right.