We were wrapping up a project, which was building a small pre-engineered metal building in an industrial facility. The only thing left was the roll up door. The building was a kit, so unlike many buildings we built, the roll up door was our responsibility. I preferred a subcontractor for this task, but that wasn't the situation this time.
I'd watched doors assembled a few times, so it wasn't like I was completely ignorant. Still, I spent some time reading the instructions and followed the steps as explained. It was fairly simple, but there were some warnings about dangerous things that could happen.
After fastening the brackets, we placed the heavy sectional door on the brackets and proceeded to start tensioning the spring, which is designed to offset the weight of the door during opening and closing. I was concentrating on keeping the pipe wrench from getting loose and smashing my hand against the frame, or whipping around and hitting me in the face. Since I was on a scaffold, my attention wasn't on one of my helpers.
One caveat in the instruction was to NEVER remove the banding around the door, until the spring was tensioned. Since the door weighed hundreds of pounds, removing the band would allow it to unroll, which wasn't a good thing; especially if you're under the door.
I didn't like the method we were using to tension the spring, so I loosened the spring and climbed down to re-group. I felt we needed to have someone on the other side to help in the operation, so I was in the process of doing just that by moving a scaffold to the other side and finding another wrench. I didn't pay much attention to the helper, who was asking about some tin snips.
As I moved about around the door, the helper found the tin snips and with the best intentions to be productive, snipped the band around the door. Instantly, the door unrolled and slammed against the floor - missing me by inches. I was speechless for a moment and then looked at the helper. I didn't need to say anything. The sick look on his face meant he realized how bad he screwed up.
Anyway, instructions are important; so is communication. If I had spent a few moments explaining the entire process, the helper would have had a better feel for the importance of not removing the band. Not that this would have prevented the accident, since the helper wasn't the sharpest knife in the drawer, but at least the right methods would have been applied.
You live and your learn. In my profession, learning can be lethal.