People share the moment they realised the person they were dating was an 'absolute idiot'

Once you were so in love, several people had to stop following you across various social media channels.
Now, the spark has gone, and you're sat opposite that once-special person and every second in their presence is agony.

It could be the discovery of a disgusting habit, or even (heaven forbid) a secret spouse, but in most relationships, the time comes when the so-called 'honeymoon period' comes to an end.
Or, to put it bluntly, that moment when you realise, not only that you and the person you've been romantically involved with are incompatible - but they're a bit of a wally.

When 'erect' can be used to describe lots of things

"She flipped out because her third grader came home with 'erect' on a spelling list, and was on the verge of calling the school.
"She honestly had lived her entire life not knowing 'erect' had any other use aside from describing a penis. - Iscariot

When your cat comes across better than you do

"I had been dating this girl for a few months and it was Christmas time. We weren't super serious but it was serious enough that I was buying her Christmas presents.
"I found something for her that was perfect and it had a connection to some funny event involving her and my cat. So I made the present from the cat. I thought I was being cute and she would make the connection.
"Instead, she got p***** my cat got her a present and I didn't. I thought she was joking.

When, well, this happens

"She got me Mad Libs [a word game] and when it was her turn to do a noun she asked, 'What is a noun?'
"I said, "It's a person, place, or thing.'
"There was a long silence as she thought. It went on for so long that I thought she must be thinking of the best noun I had ever heard. Then she said 'place.' - tedave123

When you can't bear to break this sort of news to them

"When I saw her water her plastic plant for the third time."- SoBeefy

When the north / south divide is too great to overcome

"I was on a hike and was following a trail I had read up on online. We get to a fork and I say 'okay now we need to go north.'"
"She says 'haven't we been this whole time?'
"Confused, I look at her and ask why she says that. She replies 'north is the direction in front of you yeah?' - Tork260

When you have the better grasp of physics

"She tried to argue that we could get to the moon easier if we just built a spaceship that could go underwater, and flew it through the ocean to the moon during the day time rather than straight up." - Blindjakefromstate

And when this happens

"She thought Scotland was its own island, as in separated by water. We live 30 minutes from the border." - Halfjaked

This too

"He insisted that women cannot be doctors, only nurses (and vice versa.) He said that the two are the exact same thing except one is male and one is female." - Tri_Sara_Tops

This too

"I dated a guy for about a month until I found out that he didn't realize that women's breasts made actual milk to feed their babies. He thought 'breast feeding' was just a way to hold a baby while giving it a bottle.
"I told him he was an idiot and he said, with a disgusted sneer, 'I didn't know that because I have never known any woman, who had or would, breast feed their child.'" - Star90s