“Infinities are weird” might satisfy your criteria. It surfaces in such things as - why do we consider 0.9̅ and 1 to represent the same number? - why are the

“Infinities are weird” might satisfy your criteria. It surfaces in such things as - why do we consider 0.9̅ and 1 to represent the same number? - why are there ‘as many’ even integers as there are integers, and ‘as many’ integers as there are natural numbers? - why are not all infinites equal? If you don’t know/accept these, you’ll keep using intuition that served you well for many, many problems on finite sets on problems involving infinite sets, with disastrous results.