Back in the day, I used to do a lot of CAD drafting.
There is a well known phenomena whereby your ability to see mistakes, errors, omissions or plain nonsense in your drawing diminishes sharply after working more than a couple of hours on the same drawing or draft.
Our engineering department even had a policy, that required every drawing, no matter how simple, had to be reviewed or "Redlined" (used a red marker on a printout to circle mistakes) by at least one peer drafter before submitting it to your supervisor, This saved a lot of embarrassment to everyone, especially the company (it saved a lot of money too) since simple mistakes were caught even before hitting version control or structural assessment, even production line assembly, let alone the final customer.
Most of the errors caught by peer re are simple, basic omissions like a missing dimension, missing notes boilerplate required by specification, etc. Sometimes glaring errors are caught like personal work-notes or calculations making it to the final document, duplicating sections of the drawing, etc… however the drafter that had been working on the drawing was simply "blind" to the error until someone else pointed it out.
Also, incidence of this is noticeably higher when working on a computer (v. gr. CAD Drafting) vs doing "old school technical drafting" by hand on paper using a drawing board.
Quite frequently, simply printing or plotting your CAD draft to paper, will make you see and catch some errors, invisible to you on-screen, but a "fresh pair of eyes" will catch some that you yourself can't see even on paper.
This also known to happen in other professions that require working for extended periods on a computer document, for example, Software programmers, lawyers, technical writers, etc.
In spanish we call this phenomena "Ceguera de taller" literally "Workshop blindness" however my research (below) shows it does not translate well to english.
I thought of "Mental fatigue" however I don't think it conveys the same meaning, since mental fatigue reduces your overall awareness and acuity, and you would be blind to other's mistakes on a similar drawing or document, however (surprisingly) if you look at a document that you didn't work on yourself, you'll notice immediately any mistakes even if you have just committed them on your own work.
I came up with 3 alternatives for a literal translation (Workshop Blindness, Drafter's Blindness, and Drafter's fatigue) and tested them using Google's N-gram viewer, but none of them yielded any results, so they are probably meaningless or nonsense. Ceguera de taller, however, is widely used in spanish since around the 70's which matches the time period when PCs became affordable.
so, in short:
What would be a well understood term or idiom for the inability to see your own mistakes or omissions after working for some time on the same document.
Many thanks to everyone for all the wonderful and enlightening anwers! I tried several in actual conversation and tunnel vision required the least explanations, although several ideas here work in different situations. Like "Im having tunnel vision issues with this document” when explaining I am having the condition, or the very friendly, funny and kinda ice-breaker captain obvious! Again, thank you to this wanderful community