Agree the contract before the first line of code
The worst bug I have shipped was not in anyone's code. Two systems each behaved perfectly and disagreed about what a payload meant, so data deleted itself silently — no error, no log, nothing broken. Now every shared boundary gets a written agreement first: the shape of the data, the required fields, the strange cases, and who owns what. It lets two people build in parallel and still fit together on the first try.