If you ask a cop working undercover if he is a police officer, he is legally bound to say yes.
This one has to at least be nominated for thread winner. Come on. 
The husband taking on his wife’s pregnancy sickness.
Nah, that’s at least a common urban legend. Some of these are way more bizarre.
There once was a family on “Family Feud” who, after much discussion and deliberation decided that the country they would most like to visit on vacation would be … Europe.
I once worked with a gardener from a semirural area. I described the duckbilled platypus to him (fur, egg-laying, poison claws, etc.) He said, “There ain’t no such animal!”
I’ll see that and raise you my high school Algebra II teacher who was amazed to learn that pi was not exactly 22/7.
Someone once told me “most serial killers are gay. Look at John Wayne Gacy and Jeffrey Dahmer.” When I told him adoption is considered a factor in creating a serial killer and mentioned Charles Manson, he said “Well, Manson had a good childhood.”
And someone once said in all seriousness “Marriage is for monogamy and gays can’t be monogamous. So gays can’t get married.”
Maryland?
Not so much “stupid” as “widely believed and dangerously inaccurate.”
There are a lot of similarly wrong beliefs. Like “Catholics and Protestants in Northern Ireland have been killing each other for centuries.” No… on the whole, there was very LITTLE sectarian violence until the 1970s.
Or “Serbs, Croatians and Muslims have hated each other for a thousand years.” No, actually, they all mixed and generally got along fairly well until quite recently.
Someone once said that adoption leads to becoming a serial killer.
Well, more than a few gays are saying the same thing. Not all of them are excited about getting to settle down monogamously. To a LOT of gays, that sounds more like a trap than like progress.
My husband says the same thing. It’s not that he’s ignorant of the origin of store-bought eggs.* It’s that (in his experience) the chance of accidentally coming across a fertilized egg is much, much less in store-bought eggs. A few bad experiences as a child, and free-range organic eggs are ruined forever.
*Though we do pretend that Safeway eggs come from a factory, and anyone overhearing us might think we were deeply ignorant.
That Catholics aren’t Christian.
No one is forcing them.
Sorry, NOTHING is in Maryland. 
This woman was completely serious and after my friend and I erupted in gales of laughter, we did explain that ALL eggs come from chickens. She was incredulous.
I hear you on the fertilized eggs. I don’t keep roosters. Which leads me to another stupid thing someone said: “I thought you needed roosters for chickens to lay eggs…??”
Um, no.
During a pub quiz:
Q: Where is the alimentary canal
Teammate’s suggestion: Is it Venice?
What, couvade? Sympathy pregnancy is a real thing. It doesn’t mean the wife loses symptoms, of course.
In sixth grade I went to war with my geography teacher because he declared Central America to be a country. He claimed all those lines on the map were just states. I brought in the encyclopedia entry on Panama. He sent me to detention. I am still furious.
To be honest, I had to look that up. The fact that it had no capitalization made me think it had something to do with biology. I don’t believe I’ve ever heard the term before though; wouldn’t most people just refer to it as the digestive tract?