Goldman Sachs did not have a losing day last quarter in its trading operation. I have read many conspiracy theories around the blogosphere about how Goldman achieved this feat. Most either say Goldman knows where the market is going because they get to see all the customer's orders or that they manipulate the market. The truth is a lot less impressive.
A lot of what Goldman classifies as trading is just customer order facilitation. For instance, if a customer wants to buy a large number of options they can't go directly to the exchange, so they go to the Goldman derivatives trading desk. Goldman then sells them the options and hedges out the risk. Goldman makes a small spread on the trade and that counts as trading revenue.
Goldman is also a bond dealer and that is a license to print money. The corporate bond market is a dealer market. A buyer and seller cannot come together as there is no corporate bond market so they have to go through a dealer. It is inexplicable why in this day and age of technology there is no corporate bond market that bypasses the middle man but that is the case.
With a steady stream of income from these and other customer trade facilitation businesses the company gets a huge head start every day on trading revenue. I am certain that the pure prop desks have a much lumpier record.