#1 Service Panel Issue
Many of the older homes I inspect have had some variation of upgrade to the electrical system. The lighting is upgraded, wall switches and receptacles are replaced or perhaps Ground Fault Circuit Interrupters (GFCI) are added.
Inevitably, something changes in the service panel because of the upgrade. A breaker is almost always taken away, and rarely is it replaced. When breakers are removed from the panel, a void is created. This void is the perfect size for screw drivers, fingers or rodents to find its way into.
The big box department stores sell "knock-out" or "twist-out" replacement covers that perfectly fit right into the void left by the removed breaker. These small plastic covers do protect against inadvertent contact and potential electrocution. Due to the danger electrical panels pose to untrained people, I recommend hiring a licensed and competent electrician to place these for you.