Personally, I am all for redundancy, but there is only so far you can go. Most sites have redundant power backups, redundant network connections, etc. However, none of that does any good if a disaster destroys everything in the area. For instance part of the authorize.net system is done over the phone - backups are great, but they can't get around the physical destruction of all the lines in an area. Colocation is one possible solution, but considering the amount of data that has to be available to process these transactions, and the inherent security issues, it might simply not be possible to have multiple copies of the database synchronized at multiple facilities.

TL;DR: It happens, and it isn't always avoidable.

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The best way to learn anything, is to question everything.