A Test Proves One Thing
A green test suite feels like a broad statement about your code's health. It's actually a narrow one: these specific inputs produced these specific outputs, today. Confusing the two is where false confidence comes from.
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Tested Is Not the Same as Verified
A test that runs and passes tells you the code did what the test checked. It doesn't tell you the test checked the right thing. That second question is easy to skip and expensive to skip.
The Test You Didn't Write
Every test suite has a shadow: the tests that don't exist because nobody thought to write them. That absence doesn't show up on a coverage report, which is exactly why it's where the real risk tends to live.
Deprecate Like You Mean It
A deprecation notice that nobody acts on isn't a warning, it's decoration. The gap between marking something deprecated and actually being able to remove it is where most interfaces quietly calcify.
The Contract Is What They Rely On
You decide what your interface promises. Your users decide what they depend on. When those two differ — and they always do — the second one is the real contract.
Version the Promise, Not the Code
A version number is supposed to tell callers something. Too often it just tells them the code changed — which they already knew, and which doesn't help them decide whether to worry.
Every Error Is a Design Decision
Error handling gets treated as the cleanup after the real work — the branch you fill in to make the compiler happy. But what a system does when something goes wrong is part of what the system is.
The Failure You Planned For
A failure you anticipated is an inconvenience. The same failure unanticipated is an incident. The difference isn't in the event — it's in whether the system had somewhere to put it.
Write the Error for the Person Reading It
An error message is written once, in a moment of frustration, by someone who already knows what went wrong. It's then read by people who don't — often at their worst moment, with no other information to go on.
One Wrong Default, Times Everyone
A bug in a rarely-used option affects the people who chose it. A bug in the default affects everyone who didn't choose anything — which is usually almost everyone. The blast radius of a mistake tracks how many people never had to opt in.