You have the wisdom and discipline (and steel-trap memory, apparently :) to make a mistake once and never make it again. I saw long ago that I had not. One of the lessons I drew from that observation was to program defensively. Even when I have absolute control of both code and data, however simple, I find the effort of defensive programming is usually, in the end, repaid.

Defensive programming is all well and good when dealing with data from an unknown or untrusted source. When the data is just four characters you've written yourself; defensive programming is overkill.

It's been observed time and again that one is oneself the person best equipped to lead one down the garden path.

What you've contrived as an example is a novice mistake. It's a good mistake to learn from.

I should have made it clear in my initial comment that it was not intended for you; I think you are well aware of the issues involved. It's hard to contrive an example of a novice mistake that's not... well, contrived, but please be assured that the example in question was directed not to you but to novices.