This reminds me of an experience debugging a network protocol implementation - specifically AppleTalk NBP for other ancient people. I had coded everything but my packets were rejected (aka silently dropped) when real (aka Apple implementation) packets were not. I had a copy of the good and bad packets on the screen of my computer and had gone over them byte by byte to find the problem. And there was none. From start to finish they were exactly the same, with correct check sums etc. It was time to go home and I decided just to print the stupid things to look at later.

As soon as I printed them, the error was clear. My version ran to two pages and the good implementation one page. I had not been careful to clear the buffer before sending the data (mbufs don't you know).

This still cracks me up.