The bentover barbell row isn't for beginners or those with lower back issues. (If that's you, start with the chest-supported row.) Sadly, it's one of the most butchered movements in gyms.
Good Bentover Row
Doing it properly involves going through a full range of motion as you brace your core and pull the bar to your lower chest while having a bend in the hips. Most beginners lack the motor skills necessary to do this task effectively at first, which is why I recommend starting with a chest-supported row.
Bad Bentover Row
You'll often see people getting over extended, forgetting to squeeze their shoulder blades, and rounding their upper back in an effort to grind out a couple more reps. This generally looks like a really bad version of a clean high pull.
The most common rowing mistake is when lifters short-arm the movement by pulling to their hips or stomach in an effort to lift more weight than they can handle.