My experience with coding is that stupid obvious mistakes are way more likely than 1/10000. You write something slightly wrong, keep reading it as if it were right, and that's that.

Determining if a number is prime is a bit of a nice case, I suppose, because it's so amenable to testing. The structure of the mistakes you make is unlikely to match the structure of primes, so you'll catch any mistakes more easily.

I'd still consider doing it 10000 times to be extremely difficult. Just adding 10000 six-digit numbers by hand, even with some cross-checking, is quite difficult.