Too many men on defense

[airraider]

First play of game this past Friday.

The ball snaps and a flag is thrown and whistle is blown.

The defense had 12 men on the field.

The white hat said they are not supposed to allow the play to start with 12 men and if they catch it, there should be no flag.

I asked how would there ever be a flag for 12 men if they were supposed to get them off the field, and he said if they did not catch it in time there could be a flag.

I was completely mind blown.