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

On April 24, 2001, the NYISO deployed code changes affecting Class 1 PURPA
transactions in the billing system and modified bid status flagging in the
MIS.  These changes impacted settlements for 4/10, 4/11, 4/13, and 4/18 -
forward.

There was a code design problem with the PURPA code change that resulted in
an error in the computation of balancing energy for some class 1 units that
resulted in their purchasing their entire day-ahead schedules in the
balancing market.  There was also an oversight in the implementation of the
modified bid status flagging in the MIS such that those bids were not
transferred into the billing system, impacting bid production cost
guarantee settlements for the units with a modified bid status.

The NYISO will be issuing a supplemental invoice for April 2001 settlements
on Tuesday, May 15th to adjust for the aforementioned issues.  Funds due
the NYISO for this supplemental invoice should be transferred into the
NYISO Clearing Account by close of business Wednesday, May 23, 2001.  Funds
due Market Participants from the NYISO for this supplemental invoice will
be transferred into Market Participant accounts by close of business
Friday, May 25, 2001.

PLEASE NOTE THAT: Funds due the NYISO for the April 2001 settlements which
were invoiced on May 7, 2001 should be transferred into the NYISO Clearing
Account by close of business Wednesday, May 16, 2001, as scheduled.  Funds
due Market Participants from the NYISO for the April 2001 settlements
invoiced on May 7, 2001 will be transferred into Market Participant
accounts by close of business Monday, May 21, 2001, as scheduled.