This furnace failed its carbon monoxide test. Unsafe levels were found in the house air which indicated a crack or hole in the heat exchanger.
The black deposits shown in the photo below are carbon. When you see carbon deposits on a furnace it is almost a sure sign that carbon monoxide is a problem.
Many home inspectors do not incorporate carbon monoxide testing into their inspections. This is an item that goes beyond the ASHI Standards but is important for safety. The recommendation was to replace this 25 year old furnace.