Final Edit: There are two new posts on the topic. The first is a more coherent summary of the information in this post. The second addresses the NYSE's official line on the story.
Edit: Looks like that was the official closing price on NYSE, not afterhours. However, no trade at that price is showing up in time&sale data between 13:59 and 14:00 for me.
Edit 2: Comment on ZH claims the 106.46 trade was worth $7M.
Edit 3: Bloomberg claims NYSE Euronext is cancelling these trades.
Edit 4: Found the T&S time range thanks to the Bloomberg article.
Edit 5: There's still a 106.46 bid for 100 listed under NSDQ in the order book. Edit (again!): Same order up to 800 shares now.
Edit 6: Did some addition by hand and got just under $500M USD in actual executed trades at 106.46.
Looks like there was an after-hours flash crash in the SPDR S&P 500 ETF SPY…printed low at 106.46. Spent awhile digging through T&S data and can't find it in Power E*Trade Pro. Still no official word from NYSE ARCA and there's general confusion across Twitter.