Some basic facts you learn at school can be really shocking when you first hear them, and this was long before the Horrible Histories books. Some of the OMG moments I remember were:
Punishments in Victorian schools, such as being hauled up to the ceiling.
The Incan punishment of holding a child over a fire of chilli peppers, so they would inhale the smoke.
Swarms of locusts destroying all greenery in minutes.
The story of Abraham nearly sacrificing his son Isaac. (Yes I know, it’s a Bible story which probably didn’t happen, but in my primary school days, it was almost presented as fact, before teachers had to say “some people believe”).
Of course, a teacher would normally explain these without any emotion at all, and somehow this made them all the more astounding, that they didn’t seem bothered at all.