Done And Dusted

That beautiful moment when you're feeling like a genius after writing a slick function that crunches numbers and saves results to a file. You test it again with fresh data, feeling confident... and watch in horror as it overwrites everything you just computed. Turns out you forgot to append instead of overwrite, or maybe you hardcoded the filename like an animal.

Nothing humbles you faster than realizing your "reusable" function just nuked hours of work because you opened the file in write mode instead of append mode. One character difference between 'w' and 'a', infinite difference in suffering. Should've used a timestamp in that filename, but hindsight is 20/20 when you're staring at an empty file.