Continuity mistake: When Elizabeth's maids lace up her corset, obviously both the back and front laces were pulled very tight, as this is the reason for her discomfort and eventual lack of air. After Jack rescues her, as she is on his shoulder we see the two sides on the back of her corset are far apart and the laces in between extremely loose. Yet, when she is lying on the dock, Jack slices the front laces on her supposedly constricting corset so she can breathe again...
Picture added by Super Grover
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