I have trouble understanding a programmer who takes a couple samples of a method's behavior and feels confident enough to trust that as the actual behavior of the function. At least guard you're flippant assumption with a check or something!
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.
