This occurs in Justice For All - Case 4: Farewell, My Turnabout: Trial 4-2
Warning: This is the part where you MUST save your progress, otherwise you'll be reverted to the main menu.
This is the part where there is two endings, be aware their are spoilers up ahead so you are warned, but the short story is you need to present the absolute wrong evidence and the wrong person to get the bad ending. If you do get both correct, just answer the wrong question (two wrong answers provided seems rather obvious).
The trophy will be earned when you witness the bad ending.