When Marie tells Agent Garfield the flavor of the cookies, his mouth opens in dismay. In the next shot, his mouth is closed in a grimace.
Robert would not be a candidate for the FBI. All FBI Special Agents and above have at least a bachelors degree. Robert has said in a previous episode that he didn't have a chance to go to college. In fact he would not qualify for Lt. with the NYPD with a degree. 64 College credits are required to be a sergeant with the NYPD. So even his Sergeant rank was an error.
When Agent Garfield receives the fax, he immediately frowns upon receiving it. The fax was lengthy, so it should have taken him some time to read the fax and then react. The actor likely reacted quickly just to keep the show moving.