Specifically a bug in that constant sp, which should have a 1 in bit 19 like it does for the other primes ≤ 23. This is step 1 of the algorithm they're using:

Specifically a bug in that constant sp, which should have a 1 in bit 19 like it does for the other primes ≤ 23. This is step 1 of the algorithm they're using: https://en.wikipedia.org/wiki/Baillie%E2%80%93PSW_primality_test https://en.wikipedia.org/wiki/Baillie%E2%80%93PSW_primality_...