There was a time when, if selling a sizable amount of a security, one tried to get the best execution price and not alert the buyers comprising the bid stack that there is (substantial) volume for sale. Of course, there was and always has been a time when one tried to manipulate prices by slamming the bid until it was fully taken out, usually just before close of trading, an illegal practice known as "banging the close." It appears that when it comes to gold, the former is long gone history, and the latter is perfectly legal. As the two charts below from Nanex demonstrate, overnight just before 3 am Eastern, a block of just 2000 GC gold futures contracts slammed the price of gold, on no news as usual, sending it lower by $10/oz. However, that is not new: such slamdowns happen every day in the gold market, and the CFTC constantly turns a blind eye. What was different about last night's slam however, is that this time whoever was doing the forced, manipulation selling, just happened to also break the market. Indeed: following the hit, the entire gold market was NASDARKed for 20 seconds after a circuit breaker halted trading!
To summarize: a humble block of 2000 gold futs (GC) taking out the bid stack, and slamming the price of gold, managed to halt the gold market: one of the largest "asset" markets in the world in terms of total notional, for 20 seconds.
Here is Exhibit A of either market manipulation or yet another broken market from Nanex:
December 2013 Gold (GC) Futures Depth of Book
Zooming in on the drop and showing the trading halt.