An exemplary one is "don't build your own timezone database"

It's not interesting, it's not fun, it's just a process of getting complaints it's wrong in edge cases and then fixing them, over and over until no one can find another broken edge case.

You can start by writing down England is +0, Germany is +2, etc... someone's going to mention DST and you'll put in a field for switching on the Nth Sunday of month X... later you'll run into a country that uses a different rule and you'll add a bunch of spaghetti code or write a Turing-complete DSL, etc... One day someone tells you about a village where they count 17 hour days on seashells and then you'll give up.

And if your DB doesn't produce identical results to the Olson DB in all cases then you created an incompatibility anyway. Might as well just use the Olson DB.