It is very common in homes find receptacles with signs of overheating.
Most common are receptacles in bathrooms and rooms where space heaters or hair dryers have been used.
Sometimes the signs of overheating at the exterior (like in the preceding picture) is only the tip of the iceberg in terms of how much damage is lurking behind the cover.
In this case, after taking off the cover plate, extensive damage to the receptacle and the plastic box itself was evident.
This is one of the ways fires happen in homes and this home was probably lucky to not have had one.
If you have receptacles that look anything like the one in the picture---some signs of overheating---please have them replaced by a licensed electrical contractor before using it again. It may be in worse condition and more dangerous than it looks from the outside.
Notice that this one---even though incredibly damaged, was still energized and "functional."
Charles Buell, Real Estate Inspections in Seattle