Sometimes that happens if one inserts the plug into the outlet, and one of the two prongs (not the grounded one) makes contact with the receptacle's contacts ahead of the other. If you look at the plug, you will notice that one of the two prongs, the blackened one, indicates that it was this prong the last one to make contact with the receptacle.
There is very minor damage to the other prong, which means that it was already secure in the receptacle before the other prong made contact with the receptacle. This can also happen to 2-prong power-cords. However, loose points of contact in the receptacle can do the same. Did you notice this condition happening in the past, or just this time?