PRNG bad seeding and von Neumann unbiasing
Von Neumann unbiasing only works if the random source consists of independent samples. Otherwise it is not guaranteed to produce random bits. More generally, in theoretical computer science there exist objects called (randomness) extractors whose job is to extract random bits out of imperfect random sources. Usually they need access to a small number of "high quality" random bits.

In practice, seeding a PRNG is not difficult. There are many legitimate sources of randomness, and using an extractor such as a hash function, you can take a decently sized sample and extract a decently random seed. Once you have a handful of high quality random bits, you can use a PRNG to quickly extract many more seemingly random bits.