Brubaker trying to outbid Bullen
Near the beginning of the film when Brubaker was still disguised as an inmate, Bullen is at the mess counter trying to buy a half-eaten cut of meat for lunch. Brubaker suddenly cuts in and offers more for it. Bullen outbids him again, winning the piece of meat, then gets angry at Brubaker for costing him money was needed elsewhere.
This part always bothered me and seemed a little out of character for Brubaker. Why would he do something that might create bad blood between him and another inmate, especially when he's supposed to be there to lay low and observe the conditions of the place? He's lucky that Bullen happened to be a good hearted man and not a murderer.