If you only write one document

If you only write one document (as part of developing software), make it the test plan.

Do it before you start writing the code.

This applies to new features and bug fixes.

• This also becomes the spec.
• It allows you to know when you've written the code to make do everything you should.
• It will make working as part of a team easier.
• It will make code reviews faster and easier.
• It will make testing faster.
• It will make creating automated tests easier. (You can even write them before the code-TDD style.)
• It will make things easier when the customer/client/boss changes their mind or the requirements.
• It will make future maintenance and changes faster.
• It will make creating complete and accurate documentation easier.
• You are more likely to be creating technical debt that you're unaware of without this.

If you don't:

• There will be lots of guessing.
• There will be lots of unvalidated assumptions.
• There will be lots of repetition of effort. (Working out what needs to be done and how to do it.)
• More effort will be wasted on things that aren't important.
• Code reviews and testing of the code will be slower and involve more discussion and clarification.
• You are more likely to ship with bugs.
• Future changes (big fixes or adding new features) will be slower.
• Future changes are more likely to introduce other bugs or regressions.