I'd Like to Start with the Cat - S6-E15
Continuity mistake: When Chelsea and Charlie are in the coffee shop talking about how they're doing, Charlie is drinking a cappuccino-type drink with caramel or some type of syrup or drizzle on the inside of the glass. Once Charlie says something along the lines of "She's a real ball buster, I can't get away with anything", the camera cuts back and forth between Charlie and Chelsea and with each cut, the amount of caramel on the inside of the glass increases and decreases.
Answer: That would be "Corey's Been Dead For an Hour" in season 4, episode 9.
Super Grover ★