[Uberphallus]

Exactly. Here we crunch loads of data, and by a lot, I mean our production databases run on a three digit cluster of four digit terabytes of data. Good luck having that scale in preproduction environments.

We have loads of redundancy, and dedicated "test in production" machines/datacenters to test actual production loads in actual production-scale sets machines.

Now, tests in production usually involves a pair of hands and 2 other people looking behind (dev + ops + dba), and requires a well defined rollback procedure and post-mortem. We still have absurdly high SLA (99.999%).