1) Mine is the pensieve! Just doesn’t make sense when It comes down to law enforcement.
2) The love potion causing the children produced to never know love. I feel like there would be 100 Voldemort’s running around because everyone would be using love potions on everyone.
3) Extra: I also don’t think weaker wizards should be able to cast a spell on a more powerful wizard. Like a 17 year old who finally can cast avada kadavra at dumbledore behind his back should have no effect