"at this scale even very unlikely bugs do show up with some frequency" exactly! In the past I've worked on systems where "this will never happen" occurs because

"at this scale even very unlikely bugs do show up with some frequency" exactly! In the past I've worked on systems where "this will never happen" occurs because "this will never happen" really means the probability of this occurring is extremely low. But when you're dealing with millions of possibilities that low probability event starts occurring every day. And when it occurs for one of your largest customers it's no longer "this will never happen" but "why is this happening?"