Back in late 2013 and  early 2014, the gold (and silver) market was stunned by a series of massive, unprecedented "stop" or "velocity logic" sales which sent the price of the precious metal crashing so furiously they halted the entire gold futures market anywhere between 10 and 20 seconds.  Some examples:

September 12, 2013: Vicious Gold Slamdown Breaks Gold Market For 20 Seconds

October 11, 2013: "Stop Logic" Gold Slam Was So Furious It Shut Down CME Trading Again

and January 6, 2014: Gold Flash Crashes, Halts Trading As "Velocity Logic" Circuit Breakers Triggered

Some said this was nothing but a fat finger, but we together with Nanex showed this was clearly a premeditated attempt to reprice gold lower, facilitated by that oldest trick in the HFT book: quote stuffing.