One Player per Hand Rule

 Rule:   There can only be one player per hand.

Player A shows his hand to Player B and player B makes a comment about the hand.  "You should Raise", "Hey you have a flush"  or any other comment about the action.  

One of the places that the rule gets ignored is when one player tables his hand and another does not.  It is player A's obligation to read the tabled cards.  The other players should not quantify the hand.  In some places the dealer will set the hand before all the cards are tabled.  This is usually justified by "the cards speak for themselves" rule.  While this is true, that rule saves a player who miss-reads his own hand.