Most complex systems don't work correctly. They just don't fail catastrophically very often. They fail in small ways instead. A lost user account, an incorrect

Most complex systems don't work correctly. They just don't fail catastrophically very often. They fail in small ways instead. A lost user account, an incorrect payment, a missing parvel, a poorly routed call. These things happen regularly - millions of times a day in fact - but out of hundreds of billions of interactions its hard to notice them. We're so used to things failing we stop noticing and think slightly broken is the same as working properly. That's kind of OK until it's your payment, or your parcel, or your account.