Probably the worst bug I ever encountered was a documentation bug. Back in 1970, I was at my first programming job, working for a very small consulting company

Probably the worst bug I ever encountered was a documentation bug. Back in 1970, I was at my first programming job, working for a very small consulting company. The contract was to convert a large system from 1401 Autocoder to System/360 Cobol. However, instead of the source code for the current system, we were given system specs, which were almost flawless. However, in final acceptance testing, one program, for one particular accounting line, kept producing results that differed from the current system. The customer would not accept this, even though my code implemented the specified calculation precisely. Eventually, after a few rounds of back and forth, I demanded to see the source code of the current system, and after much more back and forth, the source listing was produced. Looking at the source, it seemed to be doing exactly what my code was doing. There was one fact not taken into account - the present system, rather than running on the 1410 for which it was written, was running in emulation mode on their new 360. And they hadn't bought a license for the compiler. So the small accounting change they had made a few months prior had been made to the object code - the card deck itself. Luckily the programmer who had made the change had written a note to that effect on the listing - in the object code part, where he eventually sheepishly showed it to me. We then got paid. (I also once had a bug that almost got national distribution via Time magazine. Luckily they had only printed a few thousand copies before someone noticed it. That one would have gotten me fired, but for the fact that the same release also included a halftone compression routine that enabled them to push back their photo deadline by a day. The reason the bug escaped my testing was that it only occurred in pictures where the line count was of the form 4n+3 - I had tested with both even and odd line counts, but never managed to use one that produced the off by one error.)