That the FBI misinterpreted what Carolyn Arnold said about catching a fleeting glimpse of Oswald inside the front door.
Well, they clearly made that blunder since the two reports give different times for when it happened as well.
The first one said 12.25... the second one said 12.15
Even if Arnold gave both times herself, the FBI failed to verify which was the correct one. So, yes, they did blunder.