Term for being unable to see glaring errors after working for some time on a task?

Solution 1:

Consider calling it tunnel vision.

Tunnel vision metaphorically denotes the reluctance to consider alternatives to one's preferred line of thought; instances include physicians treating afflictions, detectives considering crime suspects, or anyone predisposed to a favored outcome. The common way to solve this problem is a second opinion, that is, getting somebody unrelated to the original investigation to look at it from the beginning, without the same biases and preconceptions. - wikipedia, emphasis mine

In your example, the reluctance isn't a matter of conscious choice; it's a form of cognitive inhibition caused by too much focus on particular details.

Here's an example of the term used in a context not too dissimilar to the one you described:

What to do about an excess of focus? Kamal Gupta suggested: "It is called 'Tunnel Vision'…. The solution is not difficult. Management has to take a break, like once in a quarter, to take a look at the big picture. - James Heskett, Is Too Much Focus a Problem?

You'd say that after working on the draft for too long, people can develop tunnel vision.

Solution 2:

In British English, can't see the wood for the trees

to be unable to understand a situation clearly because you are too involved in it

Cambridge Dictionary

For example:

"I've been working on this for 6 hours now, and at this point I can't see the wood for the trees"

I'm told this is "can't see the forest for the trees" in American English.

Solution 3:

I believe this could be an example of directed attention fatigue. It sounds like a perfect match for causing the mental blind spots that people develop after several hours working on a specific task. I wouldn't call this a well-known phrase, but it's rather self-explanatory and I would expect it to be understood.

The gist of directed attention fatigue is that your brain tires from constant focus, and it starts to either under-compensate or over-compensate for distractions. Under-compensating would be seen as losing the ability to ignore things and losing focus. Over-compensating would be seen in what you describe - ignoring details that shouldn't actually be ignored and forgetting to do things.

Looking at an FAQ on the matter, I grabbed the section on 'thinking' symptoms of directed attention fatigue.

We may have trouble focusing, leave things half done, forget things, lose things, find it hard to think, get confused more easily, think less creatively. Or we may get stuck on certain ideas, thoughts.

Looking more into related topics like the Stroop effect, I see an underlying emphasis on the fact that sustaining attention is mostly a battle of removing unwanted distractions, external and internal. There really aren't many people who can actually focus for extended periods of time without suffering performance-wise and developing weird mental blind spots. Most studies I found did note that even a 20-minute break could restore this mental ability and that a good night's sleep was nearly guaranteed to do so.

As to why this might be worse when using a computer as opposed to something more 'real', I can only suggest that computers tend to be placed in environments which are more distracting (denser placement of desks, the sound of typing, the whir of fans, etc.) or that computers are themselves more distracting (what with screen flicker and the internet itself.)