JSavitt@nyiso.com writes to the NYISO_TECH_EXCHANGE Discussion List:

Upon a subsequent reexamination of the data, the NYISO has determined that
hour-beginning 7 for the 28 July 2001 real-time market was mistakenly not
reserved.  There might have been a metering problem resulting in the
passing of some incorrect data.  Consequently, the NYISO is now reserving
hour-beginning 7 in the 28 July 2001 real-time market to verify the prices
in that hour.  The hour will be corrected or released as soon as possible.
The NYISO apologizes for the inconvenience, but feels that it must
reexamine that hour more thoroughly and institute corrections if necessary.