A common defect found in home electrical systems is grounds and neutrals terminated on the same bus bar in a sub panel.

Here is an explanation of why this can be dangerous. In short Laymen’s terms, if a fault occurs the breaker may not trip and metal components on that circuit (i.e. stove or fridge) could be come energized and electrocute someone who touches them. See below for a more through explanation.

Reference: NEC article 250.42.

We need to start by noting two things: (1) Current is always seeking a path back to its source, and (2) Current will take every available path it can find.  

The function of the equipment grounding conductors (EGC), that ones that connect to the ground bar in the panel, is to carry fault current. If a fault occurs with a piece of equipment, such that a hot conductor comes into contact with the case or other external metal part, any person who touches that equipment is going to get a shock. The shock can be enough to kill, but the current will not be high enough to cause the breaker to trip.  

However, with the EGC creating a path from the case back to the ground bar, then via the ground screw or bonding jumper to the neutral bar, the current in this path will be high enough to trip the breaker. This will terminate the event before the person can receive a fatal shock. That is why the ground and neutral buses are connected at the main service disconnecting means ? to complete the current path from the fault point back to the source. In this context, I am treating the main panel as the “source.” Once the current gets to that point, it has nowhere else go.  

If you also connect the ground and neutral at a subpanel, then there will be two paths for current to flow back to the source during normal operation. Current will be flowing in the neutral most of the time (unless the loads running at the moment are perfectly balanced among the phases). But with the ground and neutral tied together both at the main panel and at the subpanel, the EGC will be in parallel with the neutral wire. Therefore, the EGC will carry current. This will cause the external metal parts of each and every component that has an EGC its to become energized. You could not safely touch anything in the facility.