@jonnythan Said
This is one of the leading candidates for the election as President of the United States.
He was on a national stage, on national TV, talking about which 3 of his 15 direct subordinates he would get rid of.
The executive branch of the US government consists basically of the President and the 15 Cabinet departments that he is the chief executive of. The bulk of the President's job is to effectively run these 15 executive agencies.
He was talking about which of these major branches of his own government he would get rid of. And he didn't even know. He couldn't remember which of his 15 agencies he would completely cut off. The heads of these departments are his absolute senior staff. These are the people, and the agencies, who actually run the country. And he couldn't remember which he wanted to completely eradicate.
He gave the appearance of being an idiot and not having given much, if any, thought to how he was going to dramatically alter his branch of the federal government.
All the reasons you mention, plus his other flubs in past speeches and debates are why Perry is steadily falling in the polls. In my opinion, Perry biggest problem was his mostly unplanned entry into the race. Running for president takes a ton of preparation since you will be asked about every single issue facing the country.
This is partially why Romney will probably be the eventual nominee. He has been preparing for this since the last election.