Blogger and developer David Bolton has posted an interesting piece recalling some of his coding mistakes that were particularly difficult to find. In one case it took two days to find a single letter that had been mistyped. Another time it took three developers a month to discover that the original coders hadn't included a check for zero before a division operation, which had resulted in calculating daily oil trading figures that were $800 million different from actuality.
The third coding bug also involved a division-by-zero error, this time inherited from code a professor had written. Bolton concludes, "Here’s the lesson: Keep track of who does what to the code, and make sure that nobody’s altering major things within the software without telling the rest of the team."