It's easy to be snarky about nuclear weapons almost going off time and time again from the 50's to the 70's and maybe later, but when you look more closely it's actually an incredibly difficult design challenge.

A nuclear bomb has two design criteria that are at odds with each other; On the one hand it must under no circumstances detonate when it isn't suppposed to. On the other hand it must be easy to detonate when it's supposed to.

How do you design a fail-safe system that lives up to those two criteria?

It's a lot harder than it sounds.