Green tests mean the code runs. Not that it works.
Testing your own work is the other half of shipping. The first half was building it. The second is sitting with it like an adversary. Someone who wants it to break.
What happens with empty input. With too-long input. With an emoji. With the user who refreshes mid-save. With a 200ms hiccup. With a full disk.
Writing the feature is optimism.
Breaking it is pessimism.
Both have to live in the same head.
QA exists as a role because this is hard. Most people can't read their own sentence twice without their eyes sliding past the typo. Code is the same.
Solo doesn't mean skipping the adversary.
It means being the adversary.
Twice.
Nobody teaches this. Learned from the tenth bug shipped. And the first one caught before anyone noticed.
The trusted builders aren't the ones who never break things.
They're the ones who break their own things first.