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There are at least two things wrong with this picture (three if you count the dead mouse). Nonmetallic (NM) cable should never enter a metal electrical box or service panel without the protection of a proper clamp. Eventually the sharp edge on the entry hole will cause the cable sheathing to wear through, creating a short and a fire hazard. In addition, an open hole will let in moisture, bugs and critters and wreak havoc like gnawed wire insulation, nests and electrocuted mice.