Quote from Cutten:
Secondly, so-called "segregated" customer money, although separate from the broker's funds, can actually be pooled with other customers. So if someone blows up leaving a huge debt in their account, and this debt is larger than the assets of the FCM and their General Clearing Member's maximum liability, then the rest of the shortfall can - quite legally - come out of customer accounts. This happened in the UK (which has a v similar clearing system for futures as the US) in 1998/99 with the collapse of Griffin. I know because I had a small account there and lost about 40% of my supposedly safe "segregated account" when they went belly up.
So anyone who says that customer segregated accounts are totally safe, is incorrect. There are two clear risks which, whilst unlikely, are there and worth considering.