The fail here isn't just that you should have set up a shard 12 hours before or whatever. The other fail is that it seems (from the groups post) that you were relying on your data set only fitting in RAM but the underlying I/O system wasn't quite up to scratch in terms of being able to page in/out that additional 1GB of data.

It's not like your 66GB is hot all the time (or even big by any sort of measure), so that makes no sense to me, and that part of things needs to be further explained by 10gen and/or 4sq.