Possibly this question paints the distance between the novice and the experienced as well as anything. Novices believe things will proceed as they imagine while the experienced know a lot of the potential hidden bummers. Perhaps, they are not infinite but their number is robust.
I believe in Murphy’s Law. I think the safest position is to remain skeptical and apprehensive all the way through a project. I have found that I can cope with problems which emerge in my shop much easier than I can manage a problem on a job site. My solution was to eliminate job sites.
As I thought back over a few years I could recall quite a few ways plans can go wrong.
My MIG welder doesn’t work. Begin trouble shooting. Is it a GFI or circuit breaker box problem? Is it a bad outlet problem? Is it and internal welder problem?
I forgot some tools. Somebody changed something and I didn’t know about it. Rain, heat, wind, ice, whatever, came unexpectedly. I didn’t hear that. I didn’t remember that. The client changed the design. The client changed the schedule. Some illness/injury appears. Helper is not available - there are a multitude of reasons. A conflict develops in my schedule, again, there are a multitude of reasons. The extension ladder wasn’t tied in the truck bed and blew out somewhere on the interstate. The concrete is much harder than I’ve ever encountered. The bolt breaks. The generator won’t start. We can’t drive to the site anymore. Deep mud surrounds the worksite. Deep mud extends from where the trailer is parked all the way to the pavement. A truck tire blew out. The road was closed. There was a power outage. The part is backordered. The motor burned out. They went out of business. It took a lot longer than I thought.
“The best-laid plans of mice and men / Often go awry.” A word to the wise from Robert Burns.