Mistakes, compared to what?
Mistake, fault or error presupposes a standard of right, accuracy, correctness, or truth. How do you account for such a standard from nothing but mindless, purposeless, evolutionary, ever-changing matter in motion, which is all your brain, its by-products and everything else in the universe was, is or ever will be. And if that's is all there is, there is nothing to compare it to. A 'mistake' is a contradiction in terms from a naturalistic, darwinian premise, which by definition declaims any goal or purpose to the universe. Physical forces of chance/necessity do not make "mistakes". So where did you get this notion of a deviation from a standard by which to judge something a mistake? You just take it for granted that there is a standard, but can you justify or account for it from an atheistic premise?
That’s not necessary. It’s really simple:
God wants everyone to believe and spend eternity in heaven but some people will never be able to believe. Therefore, he screwed up.