The biggest mistake with a deadlift is confusing it with a squat. While a deadlift is a similar movement to a squat in which you sit your butt back and stay heavy in the heels. A deadlift is a hinging movement where you create a neutral spine that is parallel with the ground, and is not a squatting movement where you keep your chest up. The biggest mistakes I see with deadlifts are: people who lock out their knees, people who do not create a neutral spine with their head, shoulders, and back in alignment, meaning they look down or look up, people who arch their back and do not engage their core, and people who do not keep the weight heavy in their heels and lean forward.