The one that comes to mind isn't actually that bad.
A bunch of us drove in a caravan to a concert in Southern Illinois and were driving back at about 2am when the lead driver suddenly recognized that there was a stop sign. I'm sitting in the back of her Hyundai Santa Fe and "whump!" there's the car behind us. She starts freaking out and jumps out of the car to check the damage.
I jump out of my seat to throw on the 4-ways because this is an unlit intersection in the middle of nowhere, then proceed to go to the back of the car to check things out. I can't find any damage on her car and only a paint smudge on the bumper of the LeBaron following us. She can't see any damage but because it's really dark out she looks at me (the only car guy in the group) for approval to keep driving. Yep, we're fine, let's get going.
We get back to campus and I look again at the LeBaron in the parking lot before going to bed, the scratch I noticed looks to be an old one.
I get a call at 5:45 in the morning from the girl who owned the SantaFe, she's screaming at me over some "huge dent" in her bumper and wants me to haul my but across campus to look at it before she leaves for work. I get there and see this....

(Her car, minus the hitch)
She keeps cursing at me and asking why I'd let her keep going with a huge dent in her bumper.
So I kindly point to the two other "dents" in the bumper on either side of the center "dent". And ask why she bought the car from Hyundai with "dents" in it.
Her mouth dropped open in anger and it took her a few moments for her to realize what was going on. She just got in the car and drove to work without saying a word.
... heck, it was cold that morning and she didn't even bother to drive me back to my dorm...