Comment by dragontamer

Indeed. Furthermore, he highlights the killing of Ray Penbar's wife as a "mistake". However, Ray Penbar's wife was literally about to tell L that Light can kill using methods aside from heart attacks.

The fact of the matter remained: when Light wanted to "hide" a death from the Police, he'd use a different method. (Episode 5: Kill by Traffic Accident).

The fact of the matter is, if Ray Penbar's wife actually got the information out to L, then Light would have lost right there and then. He was forced to kill her as soon as she was discovered.

If anything, his mistake was killing the FBI team through Ray Penbar in the episodes earlier. Ray Penbar wasn't suspicious of Light... but when Ray Penbar died then the entire L investigation focused on him.