In Officer Marshal's office, if you spray the blod finder thing on the top of the cactus, there will be blood, same with the floor in Edgeworth's room.
Just like what Mia said: If you can't find a contradiction press the witness. The migght slip something up.
During the credits after completing Case 5, the touch screen will show fingerprint powder being applied and blown away to reveal the credits. After every few screens of credits, a dialogue scene takes place with a character from the game. During these scenes, tap the touch screen as though applying fingerprint powder to it, and blow it off (just like in the actual game) to reveal a hidden drawing of the character that is on the top screen.
If you insert Gyakuten Saiban (GBA) in the GBA slot when you turn on "Gyakuten Saiban: Yomigaeru Gyakuten" you can skip straight to Episode 5 (Yomigaeru Gyakuten) without having to finish Episode 4.