I don't intentionally quit unless there's some imporant thing I gotta hurry for, and even then I at least try to finish the current race.

However if I get bombarded with bombs, have mines thrown in my face like some enraged kid would throw sand on the playground, pull an Icaras on every single turn and really feel the need to ragequit, I DNF. I hit the brakes right in front of the finish line and wait the few seconds left. It's a race that doesn't deserve to be finished