"it works on my machine"

(common gotcha)

what happened

your project runs perfectly on your laptop. you demo it on someone else's machine (or the judges' machine) and it crashes immediately. different OS, different Python version, missing dependency, hardcoded file path.

why it's a gotcha

local development hides environmental dependencies. you installed that library 6 months ago and forgot. your code uses a Mac-specific path. your Python version has a feature that older versions don't. you don't notice any of this until you're on stage.

the fix

test on a clean environment before demo day. docker is ideal but even just testing on a friend's laptop helps. document your setup steps. if your demo depends on your specific machine, you're one coffee spill away from disaster. this is why you test on the actual demo machine, not just your own. make sure to restart services so you're not running stale code, and have a backup plan for when the environment still surprises you. reproducible environments start with proper version control.