No but if GS asks a counterparty (with whom they have billions in other contracts) to cancel contracts as a result of a system error, the counterparty will prob

No but if GS asks a counterparty (with whom they have billions in other contracts) to cancel contracts as a result of a system error, the counterparty will probably agree (if they don't lose as a result) knowing that GS will next time do the same. With a small party, this could just be an attempt to undo a bad deal. I think what the poster meant is that deals are just contracts and can be amended if both parties agree. For large banks with longstanding relationships, that is naturally much easier than for a trader that no one knows.