What is the most common mistake when backing up a trailer?

The most common mistake when backing up a trailer involves failing to look both ways behind you. This action is crucial because when maneuvering a trailer, visibility is limited, and there could be pedestrians, vehicles, or obstacles in your path that you might not see without properly checking. Looking both ways ensures that you have a complete understanding of your surroundings, allowing you to back up safely and avoid accidents. Relying solely on mirrors can leave blind spots that may conceal other vehicles or people, making it essential to physically turn your head and assess the entire area behind the vehicle before proceeding.

The other options also identify important aspects of backing up a trailer, but they highlight less frequent mistakes. Using a spotter is often a good practice and can significantly enhance safety, driving too fast is usually more relevant to general road safety rather than backing up specifically, and relying solely on mirrors, while an issue, doesn't encompass the critical action of assessing the environment with direct line-of-sight vision. Always prioritizing situational awareness by looking behind you is what sets apart safe trailer maneuvering from potential incidents.