Here's some of mine:
1. Go to any body shop. At the screen where you choose a color for your car, choose the dark green color on the left of the color selections box (4 colors down from black). Select that color. Now go right 4 colors, then down one color. Now press A, and it will say that your car is already that color. Weird.
2. Climb on top of CJ's garage and stand in front of the wall to the left. Now spawn a vehicle (Quadbike is reccommended) and press Y to enter it. CJ will attempt to enter the vehicle, but the wall will be in the way. Tap A repeatedly, and after a few seconds he will step through the wall and enter the vehicle. You are now inside CJ's attic. Do this again to leave.
Walk really close to any wall in the game and rotate the camera. It looks like CJ's elbow is going through the wall, lol!!!1
Too many to list.