Things that genuinely make me go WTF?
I can understand how ancient humans would kill a bear and then think "Man, I bet if I wore its coat I'd be toasty!" but how the fuck did they ever work out how to 'cure' it so it wasn't smelly and disgusting. I mean really, why would even THINK that you could?
On top of that, why would the same people then think "Hey, if I kill a cow I can use its flesh to make quite good tough armour and boots, if only I go about curing and tanning it in the right way, and then use steam and stuff to mould it".
There are loads of things like this.