Not without looking at the code first. But the first thing which comes to mind is accidentally generating RSA primes which aren't prime because the broken mult

Not without looking at the code first. But the first thing which comes to mind is accidentally generating RSA primes which aren't prime because the broken multiplication makes a SPSP test accidentally pass. Granted, finding a concrete example would probably be very difficult, considering that replacing modexp by rand() is only going to pass N as a suspected prime with probability 1/N; but the question was can it, not will anyone ever find an example.