Dale Earnhardt was killed during a crash in the Daytona 500 back in 2001. Earnhardt lost control on turn four on the last lap, and slammed into the wall.
At first, the crash didn't appear to be too bad. The death of the intimidator at the age of 49 forced Nascar to implement a number of new rules regarding safety.
There have been no Nascar deaths since those safety measures were put in place.