The problem was how the difficulty was decided -- it looked at the last N blocks that were successfully mined to decide whether it needed to up or down the difficulty. If you suddenly lost 90% of your mining capacity, it would only adjust the difficulty after several more blocks were mined, each of which would take ~100 minutes instead of the usual ~10. The system would eventually right itself, but it wasn't fast.

I can't recall if this ever became a problem in practice, or whether there was a code change to make it resolve faster.