Everything related to Aaron, who was severely crippled by a distracted driver 6 years before the show. In one infamous moment, after being put through a relatively easy obstacle course while texting or drinking and failing miserably, the thoroughly humbled contestants are forced to all look Aaron in the eye and promise to never drive distracted again.
The episode in Ever where Angelina is dismissed from the show and shipped off to psychiatric care. It makes every time the show or its fans laughed about her driving into a "Funny Aneurysm" Moment.
Just to put this into context, every single time Angelina takes her run in any of the challenges on Ever, there is a distinct lack of music or joking between Andrew and Christine. In fact, Andrew is initially very curt towards Christine, then seems to soften when Christine finally explains why she doesn't push Angelina any harder. If you watch him during Angelina's final episode on Ever, you'll see that he's just as concerned about Christine as he is about Angelina.
Crystal's forced withdrawal from Season 5 after the death of her brother-in-law in a motorcycle crash. After she leaves, there's a montage of the remaining drivers sending Crystal their best wishes. Said montage ends with Father Giles (the first person she spoke to after being given the news) sat there, weeping silently, unable to say anything.
Season 12 Episode 2 has two moments:
Nominee Mike telling Cody about how he was a victim of reckless driving and was in a coma for 7 weeks. When he awoke, he had to relearn everything from scratch and says he's lucky to be alive. This convinces Cody (who's in rehab for reckless driving) to promise to never harm anyone, and drive responsibly.
Philippe Letourneau revealing that one of the reasons he stays on the show as an expert is because he lost his cousin to reckless driving.
Adam's breakdown in the Icy Corner Challenge on Season 13. It really shows how much that crash he was in years ago affected him. Fortunately, he did pass the challenge.