With no offense meant to all the kind and down-to-earth people named Karen out there, "Karen" has become the internet's favorite slang term for a specific kind of person, usually a middle-aged woman, who is rude, entitled, and always demanding to "speak to the manager." If you have ever worked in the service industry, then you definitely know what I'm talking about.
Someone asked Reddit: "What is the most Karen thing a Karen has even done to you, personally?"
1.) From deadlysyntax:
Left her card in the eftpos machine at the McDonalds I was managing as a teenager, rang me to say I needed to drive it out to her, that I ruined her night, ruined her family's night, owed her free food and that she'll be laying a formal complaint when I refused all of the above. She showed up 3/4 of an hour later with steam blowing out her ears asking for her card back. Last we heard from her.