My first Python script spat out a number I didn't ask for, and it was actually right
I wrote a little loop on Tuesday to add up my grocery receipts for the week and it printed 47 when I expected 42, so I checked every receipt by hand and the script had caught a $5 charge I completely forgot about.
My bank app rounds everything and it drove me nuts for years. Last month I started dumping my card charges into a plain text file every Sunday and running a quick script over it. Found out I was paying for two streaming services I forgot I even signed up for. The script did not care about my feelings, it just added the numbers and there it was. Now I trust the math more than my own memory, which is kind of sad but also saves me money. Keep the loop simple and check it against your bank once a month, it will catch stuff you swear never happened.