If it happens even one time while you are in the middle of trying to resolve some "emergency", then it has happened one time too many. Customers don't care that you weren't thoughtful enough to keep a mirror of everything you were using.

I remember my first brush with this when Maven first started getting popular. Apache had to move their servers to a new datacenter and the server that had the drives for the main maven repo was lost in transit. It took five days to bring everything back up. During that whole time, nobody could build anything unless they had local copies.

Ever since then, I'm pathological about ensuring that I have a local mirror of all my dependencies.