@gabrielesvelto I went to a lecture in the early 1990's by Tim Leonard, the formal methods guy at DEC. His story was that DEC had as-built simulators for every CPU they designed, and they had correct-per-the-spec simulators for these CPUs.

At night, after the engineers went home, their workstations would fire up tools that generated random sequences of instructions, throw those sequences at both simulators, and compare the results. This took *lots *of machines, but, as Tim joked, Equipment was DEC's middle name.

And they'd find bugs - typically with longer sequences, and with weird corner cases of exceptions and interrupts - but real bugs in real products they'd already shipped.

But here was the banger: sure, they'd fix those bugs. But there were still more bugs to find, and it took longer and longer to find them.

Leonard's empirical conclusion is that there is no "last bug" to be found and fixed in real hardware. There's always one more bug out there, and it'll take you longer and longer (and cost more and more) to find it.