Unless I missed something I had a hard time nitpicking this episode. I only found two things.
The Ghostbusters arrive at the power plant after averting the Containment unit disaster. They get out of Ecto-1 and Slimer sits in the driver’s seat and starts playing. The car starts to move. Ray turns around and says, “Hey, the car’s rolling” but it is Peter’s voice.
When the possessed Ecto-1 is chasing the Ghostbusters, Slimer is seen fleeing right alongside them. The high angle shot shows all four GBS running with Ecto-1 in pursuit but Slimer is nowhere to be seen in this one shot.