Revealing mistake: At the beginning of the scene where the Reverend sells the guns to Mel, Bobby is walking outside and it is rainy and stormy. But once he enters the motel room there is sunlight streaming through the windows. Then, in the next scene when he is outside with Rosie, it's stormy again.
Plot hole: In the scene in the woods right after Mel has gotten mad at Bobby he asks "Have I ever steered you wrong?" and Bobby shakes his head. Yet, it was Mel's idea to sell the drugs to Hitler's Henchmen, getting himself shot, getting Bobby beaten up and causing Rosie to lose the baby in the process. Maybe Bobby just didn't want Mel to hit him again.