How Green Was My Thumb? - S3-E17
Other mistake: When Johnny and Roy take the elevator with the nurse, they take the elevator near the nurses' station, at the south end of the ER wing, and Johnny follows the nurse onto the 4th floor to try and get her name. This is completely impossible considering the actual floor plan of the hospital, since in all of the exterior shots of the ER it's only 2 storeys.