This is Episode 9 of The Vibe Coder’s Guide to the Machine: computer science for a new generation.
Picture two windows side by side. On the left is the shop page your assistant built, showing a pair of headphones at 80. On the right is the database the page reads from, where the price now says 60. Neither window is lying. They are reporting on different moments, and nothing on either screen tells you which moment each one is reporting on.
Here is one way that arrangement comes about. You ask the assistant for a small shop page: show the headphones and their price, and make it quick. Suppose it writes a program that reads the price from the database once, when the process starts, and keeps it in a variable so later pages can show it without asking again. That is a reasonable way to make a page quick, and it takes about a line. The assistant runs the page, sees 80, and tells you it works, which is true. The page goes on saying 80, and no error appears, because no part of the program thinks anything has gone wrong.
A record can fall behind the world, as the login token in the previous episode did. The same thing happens much closer to home, inside a single app. Once the same fact is kept in two places, each copy is a belief held since the moment it was made. When the fact changes and only one copy hears about it, the copies disagree. Often the cost is a stale price on a page. When a program reads a copy, works something out from it and writes the answer back, the cost can be larger: a change someone else made disappears, and every part of the system reports success.
