People sometimes ask where the discipline comes from — the three-line rule, the habit of trusting nothing except my own running code, the assumption that production will surprise me. None of it is theory. Each piece came from a specific failure that reached real users and made me change how I work, permanently. Here are three I still think about. All three, I noticed later, are things I didn't do.
1. The collation I didn't know about
This was early. I was working on a content platform for Japanese feature phones — an official carrier service, the kind where charges land on your phone bill and the carrier has to approve you. User IDs were case-sensitive by spec. userA and usera were two different accounts.
The database was MySQL. The user lookup was a WHERE on a varchar column.
What I didn't know — what I didn't know was even a concept — was that the default collation made that comparison case-insensitive. WHERE id = 'userA' matched usera too. I had never set the column to a binary collation.
