I don't know if this is what you are thinking of, but in Act 1, Hamlet is speaking to Horatio in scene 4, and in that speech he is commenting on the flaws in human beings that can bring their downfalls. This is suggestive of the idea of a "tragic flaw" that most tragic heroes have in tragic plays like this one.
Hamlet offers up three suggestions as to where the flaw can come from. He says that the tragic/fatal flaw can come from their birth -- "wherein they are not guilty" because no man can control what he is born with. The second source of flaw could be, "by the o'regrowth of come complexion." This is in reference to the balance of the humors in the body that the people of this age and earlier ages thought controlled the temperment of people. The humors (like blood and bile) had to be in balance, or a person's nature would be out of sorts as well. The third source of the flaw is "by some habit that too much o'erleavens the form of plausive manners." This is probably the most common source -- a person's flaws come their bad habits -- excessive pride, excessive temper, etc.
Hamlet's conclusion is that it only takes a small flaw to overtake all the goodness of a person. This is exactly what happens in most tragic plays. The tragic hero has a flaw, and that flaw causes his downfall, and most often the downfall of those around him. Shakspeare probably knew that we would be on the lookout for Hamlet's fatal flaw -- , after all we know the full title of the play is The Tragedy of Hamlet, Prince of Denmark.