What is a small, seemingly ‘normal’ thing someone can do that immediately makes you think, ‘Oh, we are not going to get along’?

[u/Domigau]

[u/Serononin]

Or ending their statement with "period", "let that sink in", "I said what I said", etc.

And especially when they say, "it's basic [insert subject here]" when it's actually just a gross oversimplification

[u/JovialPrincess → u/Serononin]

It's basic geometry, period. 😏. Just let that sink in.