In everyday life
A cake recipe works in your oven and burns in a friend’s: their oven runs hotter and their flour is different. The recipe was the same; the kitchen was not.
One case
A developer in Berlin builds a “today’s trips” report for a ride-sharing app. At 23:30 on 3 March, the laptop, set to Berlin time, counts 120 trips. The server runs on UTCThe world’s reference clock. Servers often run on it, so their “today” can start at a different hour from yours., one hour behind Berlin in March, and reports 117 trips: the 3 that started between 00:00 and 01:00 Berlin time count as 2 March.
Every case
A program depends on far more than its own code: the language version, the installed packages, settings, secrets, the data, the clock, the operating system. A laptop collects years of small changes that nobody wrote down. Each difference is a place where “it works for me” stops being evidence. The closer the test machine is to the real one, the more a passing test means.
The word
Engineers call this
Write down everything the code depends on, and make the test machine match the real one. Trust a result only from a machine like the one your users reach.