In the end, when Charlie is advancing on Emily while turning his light on and off, the final time he turns it on, Kathrine is in front of Emily. How could she get there so fast without being heard? There's at least 4-5 inches of water that would make sneaking very hard.
During the opening credits, there are hidden messages within the names of the people who worked on the production, which are bolded out in red within the person's name. There are five of them, the words 'man', 'cat', 'see', 'red', and 'me'. All these have relevance to the plot.