Most harvest repairs are straightforward: install a new drive belt, fix a leaking hydraulic hose, or repair a gearbox that decided to imitate a hand grenade. The damage is obvious and the repair is straightforward. The tricky repairs are the ones that require diagnosis, especially if the issues are electronic, and especially if they're on a hydraulic system that is electronically controlled. Those are the ones where tunnel vision can be a problem.
Tunnel vision is when the outward symptoms are so blatant, so obvious, that you KNOW what's wrong without even doing any checking. Let's say oil is leaking from a valve. Pretty obvious what the problem is, so you rebuild or replace the valve, go back the field, and...the valve starts leaking again. So you rebuild it again, maybe even replace it again, and it KEEPS LEAKING. That's where tunnel vision about diagnosing problems becomes a problem.
You become so focused on what you see, or think, is wrong that you don't step back and look at the big picture. Speaking from experience, tunnel vision on the aforementioned hydraulic leak could keep you so focused on the hydraulic oil dribbling onto the ground that you never look at the big picture long enough to diagnose that there's an electrical control issue that's forcing the hydraulic system into high pressure relief, overloading the system and blowing seals and other components.
Tunnel vision, the tendency to focus on only one aspect during diagnosis, can keep you from seeing all aspects of inter-related problems. Trust me on this. More than once I've had to force myself to literally walk away from a machine, drink a Pepsi and pull out a tech book or call another mechanic. Looking at pictures, reading schematics, or describing the problem to someone else often breaks me out of tunnel vision-mode, helps me take a fresh look at things, and figure out where the problem really is.
The worst thing a mechanic can do is be so self-confident, so self-assured, or so proud that they're unable to ask for "help." Nobody knows it all. Cue the Beatles, with Ringo singing lead: "I get by with a little help from my friends...."