Drummer
Better Red than Dead
Good question. The OUA allows 21 players to be listed on the game sheet and skate in warmups, but a coach must scratch one player before the game...so the fact that there was an extra skater on the game sheet wouldn't have been out of the ordinary for the officials or timekeepers.
As weird as it is to say, I think a net-extra player is actually easier to go unnoticed than a wrong number or duplicate number - nobody thinks to count the players on the bench or on the game sheet before puck drop.
Actually - they are suppose to count the number of players on the bench to be sure it's the same as the sheet (there are rules for being eligible for playoffs based on games played in some leagues). They are also supposed to check that the starting line-up matches the game sheet (no longer a Hockey Canada rule but it's an OUA Rule - 5.1).
It doesn't help that Hockey Canada and the OUA Rules don't explicitly say that it's 18+2 but rather it's "20 players (including two goaltenders)". This leaves it open to interpretation that you can swap a goalie for a skater and remain under 20. It should be stated as 2+18.
However, at this level - you would think everyone had learned this by now.