The worst mistake I made was using json files as a simple database.

The project was to be delivered the day before it was even ordered, so I had to take shortcuts.

I totally underestimated the popularity of the project, so all that file reading and writing made the server slow and sluggish. It also being submitted on a shared host, made it even worse.

In the end the host shut it down. I don't recall, but I guess I had to add that database support after all, the project being that popular...