A lesson in software design: A working feature isn't always a reliable feature

One thing I've learned while building software is that getting a feature to work and making it reliable are two different challenges.

A feature might work perfectly during testing but behave differently when real users interact with it.

For example:

• A form works, but doesn't handle unexpected input properly.

• An API responds correctly, but fails when an external service becomes unavailable.

• A dashboard displays accurate information, but doesn't distinguish between current and outdated data.

• An AI feature produces a convincing answer, but doesn't have enough context to support its conclusion.

These aren't necessarily problems you notice when you're focused on getting something built.

I've started thinking about software quality in three stages:

1. Functionality: Does it do what it's supposed to do?

2. Reliability: Does it continue working when conditions change or something goes wrong?

3. Trustworthiness: Can users understand when the information or functionality might be incomplete or unreliable?

That third stage is particularly interesting to me as more products incorporate AI.

A product doesn't become trustworthy simply because it produces an answer. It also needs to handle uncertainty and communicate its limitations.

For other makers, how has your definition of a feature being "finished" changed as you've gained experience building products?