Python. Changed a function to return a tuple instead of one value in some database code. Tests pass, gets deployed, everything works. End of the month comes. Suddenly, we get a report that we're draining people's bank accounts and credit cards.

It turns out there was an untested bit of code inside the billing process that used this function. It used the function that was changed. To make matters worse, when the exception was thrown, the billing had already completed successfully, and due to another unrelated bug it would retry despite this.

So, needless to say, type safety and good unit tests are things I prioritize nowadays.