From Ellen Ullman’s Close to the Machine:

"The projet begins in the programmer’s mind with the beauty of a crystal. I remember the feel of a system at the early stages of programming, when the knowledge I am to represent in code seems lovely in its structuredness. For a time, the world is a calm, mathematical place. Human and machine seem attuned to a cut-diamond-like state of grace.

…

Then something happens. As the months of coding go on, the irregularities of human thinking start to emerge. You write some code, and suddenly there are dark, unspecified areas. All the pages of careful documents, and still, between the sentences, something is missing. Human thinking can skip over a great deal, leap over small misunderstandings, can contain ifs and buts in untroubled corners of the mind. But the machine has no corners. Despite all the attempts to see the computer as a brain, the machine has no foreground or background. It cannot simultaneously do something and withhold for later something that remains unknown[1]. In the painstaking working out of the specification, line by code line, the programmer confronts all the hidden workings of human thinking.

Now begins a process of frustration.

[1] clarifies how multitasking typically works, which was usually just really fast switching at the time of the book.