The Wizards pulled level at 3-3 in their best-of-seven Eastern Conference second round series with the victory.
The analysis of referee calls in the final two minutes said the Celtics should have had 2.7 seconds to run their final play instead of 1.7 after Washington's John Wall hit a 3-pointer to give the Wizards' their eventual winning margin of 92-91 on Friday.
The Wizards pulled level at 3-3 in their best-of-seven Eastern Conference second round series with the victory, setting up Monday's deciding seventh contest at Boston.
The Celtics-Wizards winner will open the Eastern Conference final on Wednesday at Cleveland against the defending NBA champion Cavaliers. The East champion plays either Golden State or San Antonio in the NBA Finals.
After Wall's basket, the Wizards had one foul to give without putting a Celtic on the free throw line and Wizards forward Kelly Oubre fouled Boston center Kelly Olynyk.
The whistle blew with 2.7 seconds on the clock but an extra second ticked off and was not noticed by officials or the timekeeper.
The league report noted that "Instant replay is not permitted in this situation."
Celtics guard Isaiah Thomas missed an off-balance 3-point attempt at the final buzzer.
The review also said two non-calls were costly to Washington in the final minute, ruling Boston's Al Horford committed an offensive foul on Bradley Beal with 41.2 seconds remaining.
Horford screened out Beal to set up an Avery Bradley jump shot that gave Boston an 89-87 edge, but the report said the basket should have been wiped out by the foul.
The report also says Horford should have been called for a defensive three-second lane violation with 33 seconds remaining.
That would have given the Wizards a free throw for a technical foul.