The problem is not to add some checks (asserts? No, just kidding), the problem is that you're quickly writing a little thing for a demo out an experiment, and faster than you'd expect these ten lines of innocuous code have grown in a gigantic ball of mud upon which many businesses are built.

To me the less worse way to handle this is a softened TDD, where a consequent part of the code is covered (described, checked, structured) by a suite of reasonably atomic automated tests.