Note to Macbeth, Act 1, Scene 7, lines 48-52: "What beast was't, then, / That made you break this enterprise to me?"
When Macbeth has serious second thoughts about murdering King Duncan and says that he will do no more than that which "may become a man," Lady Macbeth gives him a tongue-lashing which includes the following:
What beast was't, then,
It certainly seems that she is referring a particular occasion ("when" and "then") on which Macbeth vowed to murder King Duncan. However, that occasion is not in the play. It is true that as soon as Macbeth hears the prophecy of the witches he thinks of murdering King Duncan (see Act 1, Scene 3, lines 130-142), but his letter to his wife has no mention of any possible course of action, and when we first see Macbeth and Lady Macbeth together, shortly after she reads his letter, it is she who takes the initiative in planning the murder of King Duncan.
That made you break this enterprise to me?
When you durst do it, then you were a man;
And, to be more than what you were, you would
Be so much more the man. Nor time nor place
Did then adhere, and yet you would make both . . . .
What happened? It's possible that Shakespeare wrote a scene that later got dropped from performances and therefore from the text. It's possible that Shakespeare, who seems to have worked very fast, simply forgot some of what was in the previous scenes. It is remotely possible that Shakespeare meant us to see that what Lady Macbeth says is not true. None of these possibilities is particularly satisfactory.