There's a lot to be said about that: https://lean-lang.org/doc/reference/latest/ValidatingProofs/

But let's assume Lean is perfect: - you have to trust your file system - you have to trust your OS - are you sure your monitor displays "proof is valid" correctly - maybe there's a bug in your CPU hardware? - or background radiation flipped a bit in your RAM - do you trust your brain? Maybe you went to bed last night firmly believing "that proof is wrong" and you woke up this morning with a different belief that "the proof is valid".