When Frank Divinci goes to Manny for a final favor of organizing a car ride away he is unshaven and has clearly been living rough, the next scene when the limousine arrives not only has he had a change of clothes, a hat but now he is back to having just the goatee.
Towards the end of the closing credits when the film's logo appears and stops in the centre, the Motion Picture Association of America logo (which is the last in the credits to be scrolled) stops halfway at the top.
Before Frank pulls up to Manny's for the first time, you can see a blue sandbag on the curb, marking the spot where Jim Belushi is to stop the car at the proper spot so as to be within the camera frame.
When Det. Frank Divinci talks to Joe for the first time about the crime in the car we can clearly see the camera in the reflection of the window. As the scene progress the camera zooms in and a head can be seen next to the camera.
In the overhead shot where they pick-up the third "suspect", the shadow of the boom mic is visible in the lower left-hand corner.