There were other examples, but in general primes was very famous problem that was BPP but not P, so when P solution was finally found, those that suspect BPP = P took it as a hint it may indeed be true. Of course, it's not a proof - it's just a kind of side note, and there are still BPP problems that don't have P solutions, but it is said that their number is decreasing, so there might be indeed time when no such problems would remain. Which also wouldn't be, strictly speaking, a proof that BPP = P but for all practical purposes it would be as if it were true.
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thanks!
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Don't remember the exact lecture there, but somewhere in Week 3.