flying outside your personal envelope without an instructor beside you.
Well... I think I know the spirit of what you mean by instructor, but they have accidents too!
You're not going outside your personal envelope because there's an instructor beside you, but because you've briefed another competent pilot, and the two of you agree on the resulting envelope. Perhaps it's your envelope, perhaps that of the other pilot. I've had a couple of instructors sit beside me wide eyed, as I flew a required flight test maneuver. One later asked me to do a roll for him in the 172, I declined. He knew it was not within his envelope (very wise), Those circumstances were not within mine either.
No one plans to have an accident, so obviously for an accident to occur, something had to change, or otherwise not go according to plan. The better, and more broad thinking the plan is, the easier it is to follow, and thus less likely to result in an accident.
Do you plan for your flight to be safe? Do people plan for their drive to be safe? As a volunteer firefighter, I drive my car, and sometimes a fire truck, to car accidents. As I drive, I think to myself, "The person who jut did this before me crashed, what will I do differently?". Similarly, I used to fly into northern lakes to help recover wrecked floatplanes. That pilot just crashed while landing here, what will I do differently so I don't?".
Sometimes a few seconds of thought and planning, or at least enhanced awareness, makes a big difference in safety.