In Knives Out, Marta vomits whenever she tells a lie. It's an interesting quirk that makes it difficult for her to execute Harlan's plan while evading B. Blanc's suspicions. When she is in the study with Harlan, she claims that she cannot play Go because she had too much champagne. It seems the movie goes out of its way to show us that this was a lie: we see in a flashback that she had refused the champagne. This suggests that her vomiting quirk is fake, she does it whenever she wants. But this causes a contradiction: why would she vomit in the soda cup when no one was even there to see it?
Because Marta's lie-induced vomiting is likely a guilt-based physical reaction to emotional stimuli. She doesn't get sick because she's lying, she gets sick because she's a good person, and lying makes her feel so guilty she vomits. Telling Harlan she's too drunk to play Go wasn't intended to fool or deceive him, it was a weak excuse that she knew he wasn't going to fall for to get out of sitting down for a game she wasn't in the mood to play. There was nothing to feel guilty about, because she wasn't trying to hide anything. Just making idle protests to get out of playing the game, which was never going to work, and which she knew wasn't going to work.
Marta vomits when she directly lies. She gets only nauseous when she conceals the truth. While remaining in the lie, she gets sicker and sicker. At the end of the movie, she was able to hold off vomiting for an extended period of time. But, while facing the person to whom she was lying, she could not hold off vomiting indefinitely.