It's a little different in software. If I'm writing a varint decoder and find that it works for the smallest and largest 65k inputs, it's exceedingly unlikely t

It's a little different in software. If I'm writing a varint decoder and find that it works for the smallest and largest 65k inputs, it's exceedingly unlikely that I'll have written a bug that somehow affects only some middling number of loop iterations yet somehow handles those already tested transitions between loop iteration counts just fine. For a system you completely don't understand, especially when the prior work on such systems suggests a propensity for extremely hairy bugs, spot-checking the edge cases doesn't suffice. And, IMO, bugs are usually much worse the lower down in the stack they appear. A bug in the UI layer of some webapp has an impact and time to fix in proportion to that bug and only that bug. Issues in your database driver are insidious, resulting in an unstable system that's hard to understand and potentially resulting in countless hours fixing or working around that bug (if you ever find it). Bugs in the raw silicon that, e.g., only affect 1 pair of 32-bit inputs (in, say, addition) are even worse. They'll be hit in the real world eventually, and they're not going to be easy to handle, but it's simultaneously not usually practical to sweep a 64-bit input space (certainly not for every chip, if the bug is from analog mistakes in the chip's EM properties).