Nobody's Perfect ~ Solace Or Excuse?

"No one is perfect."
This popular phrase can reflect either empathy or narcissism, depending on the point of view and motive for the person who is using it.
When spoken about another person who has made an error in judgment, or done something hurtful or painful toward another, it can be an expression of sympathy and understanding for a person's "human condition" of imperfection, frequent mistakes, and emotional reactiveness. All of us are under that umbrella.
However when spoken about one's self, it MAY be about cutting one's self "slack", or not being so hard on ourselves. That's healthy, of course; demanding perfection from ourselves and judging ourselves harshly is not healthy.
But it can also be spoken in defense of one's actions and behaviors, in order to justify or excuse an offense one has committed.This is when the phrase is incorporated into Narcissism.

So, Sarah backs into Darlene's new car and breaks a tail light, makes a dent, scrapes some paint.

DARLENE says "Well, nobody's perfect!" and they exchange insurance cards and phone numbers. Darlene is empathizing and sympathizing with Sarah's human condition of being prone to mistakes and errors. She helps diffuse the tension, Sarah's embarrassment, and what could have been a terribly stressful incident.

During the same incident, SARAH says, only to herself, "Well, nobody's perfect." She gets out of the car, apologizes to Darlene, and they exchange insurance cards and phone numbers. Sarah is sympathizing with her own human condition and calming her own harsh judgment toward herself, diffusing her anxiety, but she's not making excuses or justifications for her error; she took responsibility for her mistake and was respectful and courteous about it.

Same incident again, but this time Darlene is visibly shaken up, the crash scared her, and she gets out of the car and runs around to look at the damage. Sarah gets out ALREADY in "defensive mode" before Darlene says a single word.
Sarah spits out in a hostile tone "Nobody's perfect!!!"
She does not apologize to Darlene, and she reacts to Darlene's FEAR and ANXIETY as if Darlene is JUDGING and ATTACKING HER.
When Darlene starts to display some anger because of Sarah's rude and blame-avoiding behavior, Sarah repeats the phrase again as if it's a defense... "What's your problem! Nobody's perfect!!!" 
On the phone with her brother, Sarah repeats it a third time, recounting her version of the incident to him in a defensive tone. "I mean really! Nobody's perfect!!!"