"If the tractor was off, he could not lift the tiller. "
You're right but I still would shut everything down at the first hint of a problem. Sitting in the tractor seat, it's hard to see what the problem is and I wouldn't take the chance of lifting a piece of equipment until I've analyzed the situation. I don't own a tiller but when I've jammed 3pt ph diggers in the ground, I approach them with caution AND the PTO AND tractor shut down. There's still a lot of built up potential energy in them even with stuff off. I've taken a shear bolt out of the auger while it's stuck and had the frame and gearbox "jump". I've got a long pry bar that I try and use instead of putting my body parts in harms way.
That being said, accidents DO happen and I'm sure we're all faced with "new" situations that we haven't seen before that have potentially dangerous consequences. As you indicated, "hindsight is 20/20". The key, IMO, is to analyze it best you can to foresee any possible dangers of your next action. Not only does that pertain to tractors, it relates to chainsaws, car repairs, house repairs, backing your car out of the drive, etc. I do a lot of work on my cars and crawling underneath one still scares me, which I believe is a good thing because it makes me safety-conscious.
Just last winter, at a barn where I was doing some work, I fell backward stepping off of a stepladder because I was hurrying and forgot I was on the SECOND step and not the BOTTOM one. That extra foot of unanticipated "air" landed me hard against a wall where there was a nail sticking out a few inches from where the back of my head slammed into it. How do you prevent that? Well, for one thing, I've taken to pulling nails out of boards when I see them exposed, whether they're on a wall or laying on the ground or whatever. Even if the next stop for the board is a dumpster, I'm either removing the nail or bending it over.