Defensive holding and ineligible receiver down field are very interesting penalty's in that both actually provide depth to the game.
Defensive holding happens when you have a bad corner on a good wide out and you leave them without a safety over top or you try and press a receiver inside without a good bump and run corner.
Ineligible receiver down field happens when you wait to long on a pass play most often on play action plays.
both examples of the right way to utilize penalty's