A 2 prong receptacle is ungrounded which means there is a potential for shock. In the event of a short, the metallic parts may become charged as no grounding wire is present to disperse the charge.
To fix the grounding issue with a 2 prong receptacle, add a GFCI (Ground Fault Circuit Interrupter) to the first receptacle in the circuit. This will protect any device past it and will react to any short, thus protecting the circuit.
-Site Supervisor, Dustin