Veteran developer and frequent contributor, Argle, once worked for a company which handled shipping. On April 3rd, 1988, a C function which used to work stopped working. What's so special about April 3rd of that year? Why, that's when the United States Post Office changed their rates.

The post office changed their rates on a fairly regular cadence, of course. The previous change had been in 1985. Thus the developers had planned ahead, and decided that they wanted to make the rates easy to change. Now, this was mid-80s C code, so they weren't quite thinking in terms like "store it in a database", but instead took what felt like the path of least resistance: they created a lookup table. The function accepted the weight of a piece of postage, checked it against a lookup table, and returned the shipping price.

Clearly, whoever had updated the table had made a mistake somewhere. The developer tasked with tracking down the bug printed out a code listing and pored over it. They couldn't spot the bug. They passed it off to another developer. And another. Each traced through the code, looking for some sort of obvious logical bug. They were all competent developers, this organization wasn't a WTF factory, so it ended up being quite a puzzle to each of them.

Finally, the listing landed in Argle's hands.

I'm making this code up, but the old version of the lookup table might have looked something like:

const int weightInOzToPriceInCents[] = {
    0, //you can't mail something at zero weight
    22,
    39,
    56,
      73,
    90,
     107,
    …
    1042//60oz package
};

Again, I'm making this code up for illustrative purposes, I'm sure they included much nicer conveniences. The core logic was the same, though: take a weight, and return the price in cents. But what I want to draw your attention to is that this particular code layout doesn't align significant figures. And, given that this was the 80s and nobody had auto-indenting tools, some of the indenting was inconsistent. Some developers had tried to align their updates in the past, some hadn't.

So, whichever developer was tasked with updating the table decided to fix that indenting. But they had a brain-fart moment. So when they fixed it, they fixed it thus:

const int weightInOzToPriceInCents[] = {
    0, //you can't mail something at zero weight
    0025,
    0045,
    0065,
      85,
    0105,
     125,
    …
    1225 //60oz package
};

Argle looked at the code listing for about two seconds before saying, "Hey, aren't a bunch of these numbers in octal?" Every number with a leading zero was in octal.

The developer responsible snatched the listing back out of Argle's hands in disbelief. "I didn't. I did. UGH."

In making the change, they had decided to take every number than wasn't already padded out and pad it… with zeroes. It seemed like a good idea at the time. Through pure luck of postal rates and previous indenting, they didn't end up padding any lines which weren't valid octal numbers, otherwise they'd have gotten a compile time exception. Everyone else looking at the code had been looking for a bug in the logic of the code itself, and ended up with "zero blindness", completely missing the octal numbers.

The developer walked away, muttering curses. It didn't take them very long to fix the code, but it did take them quite some time to recover from the embarrassment. They weren't alone, though, as everyone who had tried to trace through the code had missed it, and earned their own facepalm moment when they understood what they'd overlooked.