Look at the one min. Prem (spread between SP futures and SPX). You can find the Buy and Sell levels here: http://www.indexarb.com/index.html
acrary so what happened? the PREM got out of line and the buy program hit but there wasnt enough liquidity on the offers? why did it have to hit in such a drastic fashion? can't they buy in a more orderly way? what's the best course of action if something like that happens? stay out?