A combination of procedural errors and oversights and the design of the ferry itself caused the vehicle deck to flood, after the ferry set sail with the bow doors open.

The Zeebrugge disaster, involving the ferry Herald of Free Enterprise in 1987, was caused by a catastrophic failure of safety procedures and "human factors." The immediate cause was that the ship sailed with its bow doors wide open. As the ferry accelerated out of the Zeebrugge harbor, the "squat effect" caused the bow to dip, and a massive 4-meter wave cascaded through the open doors onto the vehicle deck. Because the deck was a vast, open space with no watertight compartments, the water surged to one side (the "free surface effect"), causing the ship to capsize in just four minutes. The deeper reasons were systemic: the assistant bosun, whose job it was to close the doors, was asleep in his cabin, and the captain could not see the doors from the bridge. There were no indicator lights on the bridge to show if the doors were locked. Furthermore, the company, Townsend Thoresen, had placed immense pressure on crews to maintain tight turnaround times, encouraging a culture where speed was prioritized over safety, ultimately leading to the deaths of 193 people.