When you bring your reportcard (with many failing grades) to your parents (or your parents find out your grades somehow) Report Car-nage is the long talk about you (and how your life is going to be if you dont blah, blah blah...)
Usualy it consists of 3 parts; the anger, the acceptance, and then a resulution plan,
1. Anger: This part is where your prents talk (or yell) to you about how they have trusted you, how they are paying for your stuff, how they are frusturated of your grades.
2. Acceptance: This part is a bit low paced and your parents talk usualy to eachother about your path that you chose for your life, that they thought you were smarter. You can sense this part from before because the transitions between two parts are quite soft. Sometimes they even cry.
3. Resulution: This part starts when you think that the conversation was over but your parents suddenly change topic. They usualy tell that you should work more, play less (pc, xbox, ps), or go out less with friends.
Eric: "dude did your parents got angry last night about that F on geometry?"
Dude: "yup, it was a total Report Car-nage"