A few months ago I happened to run into a bug that’s the perfect example of exactly the scenario @eweiss outlined in his post.
I was using Julia for extremely basic data analysis – finding the maximum value in a column of a CSV file – and discovered that the answer was simply wrong. I only spotted the bug because I manually inspected the results and the number seemed to me to be too low.
I filed an issue that outlined the bug, identified its cause, and contained a reproducible test case. In it, I wrote
The implementation is here and is tested, though the test only tests a simple special case for which the values and indices are exactly equal.