After it has been revealed that Larry Grey has drugged Tom Branson, Lord Grantham mumbles to his mother, in French, "il ne manque que ça" which Merriam-Webster idiomatically defines as "that takes the cake."
Carson makes the remark that the new footman, Alfred, thought he was serving dinner at the Hotel Metropole. He no doubt was referring to the 5 star Hotel Metropole in Monte Carlo, famous for serving royalty at the time.
The trivia item below may give away important plot points.
Mary forbids Matthew from seeing her the night before their wedding. This tradition comes from a time when marriages were arranged and neither the bride nor groom would see each other before they were standing together in the church. The fear was that if the groom saw the bride the day before and didn't find her attractive, he'd run away. The bride was even veiled to further prevent the groom to see who he was marrying. By the time the ceremony had started it was too late for him to get away.