NYSE Arca, the largest listing venue for ETFs in the US, suffered a trading glitch on 20 March 2017, affecting the closing auctions of hundreds of ETFs. The problem followed a system upgrade that morning and forced the exchange to suspend trading and cancel open orders for all listings at 4.13pm that afternoon. Four of the five largest ETFs in the world, including the iShares S&P 500 Core ETF (NYSE Arca: IVV) and the Vanguard Total Market ETF (NYSE Arca: VTI), were among those affected.

NYSE Arca has seen its market share of ETF listings drop to 75% from 90% in 2016.

The outage meant that some ETFs did not close by auction at the end of the trading day. While ETFs can trade at various exchanges regardless of where they are listed, they typically revert to the listing exchange for the closing price. The closing auction is a mechanism that determines the settlement price for the day, a figure widely used by investors to value their holdings.

The NYSE stated that 341 securities did not complete a closing auction because of the glitch. Of the 1,230 securities listed on Arca, only 53 closed via a normal auction, although a NYSE spokeswoman said that typically the exchange only runs closing auctions for 500-600 ETFs on any given day depending on demand.

The exchange invoked its procedures for determining the official closing price when the exchange is unable to conduct a closing transaction due to a technical issue. For the ETFs that did not complete a closing auction, the official closing prices were based on the volume-weighted average price in the last five minutes of trade.

Competition for ETF listings is heating up, with CBOE’s BATS and Nasdaq both vying for new business. NYSE Arca currently has around 75% of all ETF listings in the US, down from a high of 90% in 2016. In February this year, Intercontinental Exchange, the parent company of NYSE, revealed US regulators had recommended filing civil charges relating to another glitch in 2015 that halted trading on Arca for over 3 hours.