One problem that I run into while building is measuring things as I want them to be.
If I need a long course of a wall to line up, it's easy to close one eye, squint the other, and make a range of possibilities work. There I am, head down, looking along the back line of a wall block, rotating my head this way and that way, one eye closed until—hey presto—it looks like it's aligned to the longer line of the wall. It's magic. I didn't even have to adjust the block, I just had to will its current setting into alignment—or, rather, I had to will the alignment to the current setting of the block.
Obviously this is insane and wrong.
The block is how it is, and the larger alignment of the wall is how it is, and if the block and the wall are aligned then the block is correct, and if not, they're not. The block (or the wall, but the wall is so big I'm not going to adjust it, so it is de facto correct) objectively needs to be adjusted if there is a difference.
But. If it's hot. If it's been a long day. If I've been working on the wall for weeks or months. If my back is sore. Well, then subjectively that block is aligned. Then, ironically, all that work before that block is somewhat wasted, even as it's used as a rationale for making the shortcut.
The only correct answer is to pause, close your eyes, open your eyes, see things as they are, and act accordingly.
Unrelated, but made me think of the futility of objectivity, which isn't even always necessary:
So much for Objective Journalism. Don't bother to look for it here — not under any byline of mine; or anyone else I can think of. With the possible exception of things like box scores, race results, and stock market tabulations, there is no such thing as Objective Journalism. The phrase itself is a pompous contradiction in terms.—Hunter S. Thompson. "January". Fear and Loathing: on the Campaign Trail '72 (1973).