Safeharbor

The use of "sell at mkt. on close" or "buy at mkt. on close" is used most often in very heavily traded stocks to close out a position or to set up a position for the following day & is usually filled very close in price to the previous trade because of the depth of bids & offerings. In the case of ARU today it looks like the purchase order at $9.12 was completed just before the close & the next best bid was $9. Someone got screwed. Particularly dangerous practice in thinly traded stocks. You'll notice quite often bids & offerings have widened out at the close to benefit the .05 & .10 traders.

Regards

Joltin