Cake Karen

While getting Mr. Maellard's birthday cake at Top Tier Cakes, Mordecai and Rigby shift over to the empty pre-paid pick-up line due to them having a receipt in advance, giving them priority. Unfortunately for them, a stubborn lady keeps insisting that they cut the line when they very clearly didn't, and feels privileged to be served first since she's apparently waited longer than anyone else. Nobody listens to her, and it eventually culminates in her attempting to cut Mordecai and Rigby with a cake cutter for taking away what she feels are her privileges.

Example of:
Obnoxious Entitled Housewife