The number two stock exchange - NASDAQ - crashed due to "processor problems" Thursday, Aug. 22, 2013. The resulting shut down lasted about 3 hours. In the attached video, Paul Vigna of the Wall Street Journal's Money Beat interviewed Professor Charles Jones about the phenomenon. Many of the standard feeds do not give much technical information on the problem.
Perhaps the best explanation(so far) comes from Jaikumar Vijayan in his Computerworld article "Failed UTP SIP system shutters NASDAQ trading". The article provides a link to a website for the UTP SIP Plan and who may connect with the trading technology. But what about the information processors and the technology systems that control them? (For a basic description of them please see the entry in the Wikipedia.) Searching the NASDAQ site does not give much information on the technology nor of the shutdown.
Some of the answers may lie in the UTP Unlisted Trading Privileges Participant Quote Line Specifications. Page 8 of the document tells how the quote line interface "utilizes a TCP/IP protocol with fractional T1 circuits". The General Description tells that the SEC grants Unlisted Trading Privileges (UTP) on selected NASDAQ securities (to those requesting the privileges.) Digging deeper page 15 describes the number of types of Control Messages used by the trades system. And, yes, there is an Emergency Halt Message capability that is part of the guidelines.
Page 19 talks about circuit breaker events possibly leading to the Emergency Halt Message. The next document - The NASDAQ Listing Rules - explains that an emergency trading halt may occur due to "misuse" or a malfunction of the system. The NASDAQ TotalView-ITCH 3.2 direct data feed product also explains the built-in capacity for emergency shutdown messaging (see page 3). There are hundreds of pages (perhaps thousands) of technology documents related to the processes and procedures of NASDAQ trading. It seems a wonder that NASDAQ doesn't crash more frequently. More data is needed!