What are the most common dev mistakes?

Let me share with you the 10 mental models to help avoid as many defects/bugs as possible 🐞

I've been running tech dojos at Theodo for a year and a half now, and after reviewing hundreds of defects and bugs across many different teams, I've started noticing recurring patterns. From these patterns, I created visuals to give devs useful mental models during our dojo sessions.

This week, I'm sharing my personal favorite: the Broken Window theory 🪟. The idea is simple - the presence of disorder (e.g., broken windows) creates a sense of impunity for the next person who breaks one.

👉 A classic example in code:

I know this code isn't great, but I saw it done that way elsewhere so I figured it was fine

(I've heard this dozens of times)

To fight disorder in our codebases - and the bugs that often come with it - we need to maintain strict discipline and address new mistakes immediately, or risk seeing them spread via CPDD: Copy-Paste Driven Development.

If you already have broken windows, you need to address them one by one. Invest the energy to train devs, fully repair the issue, and if possible, add a CI check to prevent it from happening again.

When I join a project, here are the broken windows I usually look at:

• bug backlog > 0
• unaddressed Sentry errors > 0
• untreated errors/warnings (linter, console…)
• dead code
• inconsistent file structure
• lava layer antipatterns (unfinished tech refactorings)
• outdated dependencies

On the flip side, a clean, up-to-date, and well-organized codebase makes devs want to do things well, encourages quality, and prevents defects.

👉 Any serious effort to improve a project's technical quality must start by fixing all known broken windows, with zero compromises.