Not the most pleasant topic, but does lend to some interesting approaches to the sport.
Let's just say that all crashes are negative events and if injury results even more so.
So I'm gonna limit my response to NASCAR only. And limit the deal to effect on the season and potential effect on the sport.
The season: Would have to be Marlin at Kansas. A strong points contender is eliminated from the hunt.
Runner-up: Jr at Fontana. Though Jr is much maligned, going into this race, the tenth of the season, Jr was 5th in the popints standing with 5 top fives in the 9 races leading to California. Unknown to fans and official alike Jr's crash resulted in a seemingly sifnificant concussion. The upshot of that, it was to be the 24th race of the season at Bristol before Jr again scored a top 5. Seems he went on from there to have one of the top ranking latter thirds of the series.
To the sport:
Both of the above may prove to have far reaching impact on the sport as well.
Sterling's crash seems to have led to the serious consideration of creating a driver substitution program which could lead to some questionable driver champions in the future.
Jr's crash has led to NASCAR entering the realm of medical clearance on a more intense basis. This too may lead to many conspiracy and collusion issues in the future.
Might haves:
The BGN crash early in the race at Talledega which eliminated an astounding number of cars from contention, led me to wonder if we might see a race called off early for lack of competitors.
The Steve Park- Dale Jr crash at Pocono. Steve's car came so very close to escaping the guardrail's containment and entering a fan populated infield that my heart skipped a beat or two. Had that happened and fans were either seriously injured or killed, the consequences of such very likely would have an impact on the sport not many of us have ever seen.