In act II of Arthur Miller's play Death of a Salesman, we find Willy Loman visiting the workplace of his neighbor, Charley, aiming to ask him for money. We know that Charley is Willy's only friend and that he often helps him out with money.
As he proceeds, Willy enters into one of his hallucinatory states, and here he sees Charley as a younger man, teasing Willy about Biff's football game back in high school. Right as Willy begins to yell and scream as he did back when the actual moment happens (he is still re-living the hallucination), Charley's secretary asks his son, Bernard, to go and calm Willy down.
When Willy snaps out of it and speaks with Bernard he is amazed to see that the latter is now a successful attorney, and that he is even going to work on a case in D.C. Willy, in his desperate aim to see himself or his sons become something, eventually breaks down and asks Bernard what could have possibly happened to Biff, Willy's son, that he simply does not have it in him to succeed.
It is here when Bernard tells Willy that it all seems to have begun after Biff flunked Math during his senior year of high school, ruining his chances to attend the University of Virginia (UVA). The problem, according to Bernard, is not so much that Biff flunks Math, but that Biff refuses blatantly to get help through Summer school, and that he even burns his UVA memorabilia.
Bernard tells Willy that something major must have happened that summer that simply "shut Biff down". This triggers yet another major memory in Willy Loman which will be exposed later on in the play, during the restaurant scene. This is when Willy remembers how, during that Summer, Biff goes to Boston looking for Willy to get advice about the situation with his Math grade. Accidentally, Biff finds out that there is a woman in Willy's hotel room, and that Willy has not only denied the woman, but comes up with a ridiculous explanation for her. Additionally, Biff realizes that the woman demands that Willy gives her the stockings that he had gifted her. This is a huge blow to Biff since Linda, Willy's wife and Biff's mother, has to repair her own stockings in order to save the family some money.Moreover, the man who has "created" Biff, his own father, turns out to be less than genuine. This is the end of Willy, as Biff ever knows him.
Therefore, this is the moment that Bernard refers to, but which he does not know about: That during Biff's Senior summer, he discovers that his father is having an affair with another woman. In turn, this means that Willy is not the role model that Biff has made him out to be, nor someone who can be trusted. Hence, the incident certainly seared a serious trauma in Biff's life that has rendered him unable to trust anybody, even himself.