I almost escaped.

I almost escaped!  A bad day for this little guy.

Sealing opening in panels helps keep debris, bugs and as in this case rodents out of panelboards also help protect electrical equipment.

From the 2014 NEC

110.12 Mechanical Execution of Work

(A) Unused Openings.
Unused openings, other than those intended for the operation of equipment, those intended for mounting purposes, or those permitted as part of the design for listed equipment, shall be closed to afford protection substantially equivalent to the wall of the equipment. Where metallic plugs or plates are used with nonmetallic enclosures, they shall be recessed at least 6 mm (¼ in.) from the outer surface of the enclosure.

From the 2014 NEC Handbook-
This section requires all unused openings other than those openings used for mounting, cooling, or drainage to be closed up.

408.7 Unused Openings
Unused openings for circuit breakers and switches shall be closed using identified closures, or other approved means that provide protection substantially equivalent to the wall of the enclosure.

From the 2014 NEC Handbook-
The requirement of 110.12(A) for closing unused openings (other than those provided for equipment mounting or drainage) applies to all electrical enclosures, including panelboard cabinets, switchgear, and switchboard enclosures. An unused opening may exist as a result of a renovation or an alteration of existing equipment. These two requirements are necessary to restore the electrical equipment enclosure integrity to a condition that minimizes the possibility of an escaping arc, spark, or molten metal igniting surrounding combustible material and also minimizes the potential for accidental contact with live parts.

From Utilityproducts.com

There are many documented instances of damage to critical electrical equipment caused by wildlife activity. For example, $300,000 in damage was caused by a squirrel to the electrical equipment at a community center, causing a power surge that impacted the heating and air conditioning system and some parts of a boiler system.