XOP, for example, was tooling along going into the close around $49.39, then the closing print comes in at $51.66.
That is about four percent away from the last trade. That's a big difference for a closing print, on the last day of the quarter.
Similar strange moves for XBI and XME.
What's strange is that immediately after that last print, XOP went back to trading at $49 and change in the after-hours.
In other words, the closing print was an outlier.
It's not clear what happened. Because these ETFs all started with an "X" in their symbol, it's possible this is related to earlier problems NYSE Arca reported with symbols in the range of UTG to ZSML on Tape B, which consists of most of the ETFs.
Beginning at 10:24 AM ET, Arca reported that trading was unavailable in those symbols, apparently due to a server glitch.
NYSE reported that all trading issues had been resolved at 11:05 AM.
As for what happened at the close, it's possible there was problems disseminating market on close imbalances related to the earlier glitch. If that's the case, the market makers would not know how much to pare off, which could lead to the strange closing prices that we saw.