Harmless Poisonous Drink

What was the incident?

That's because the poison was in the ice. Since Marissa took time to drink her first, the ice melted & poison got mixed up in the drink very well. Whereas, Juliana was thirsty & finished drinks in hurry; it didn't get enough time to melt the ice & hence poison didn't get mixed up with the drink. That's how Juliana survived!