Should have used a cryptographically secure PRNG. The Mersenne Twister (for some reason still the default in many settings) has an insane period of 2^19937 − 1

Should have used a cryptographically secure PRNG. The Mersenne Twister (for some reason still the default in many settings) has an insane period of 2^19937 − 1, but after observing 624 "random" values, you can recover the internal state and predict all the rest.