I had a wonderful Heisenbug once. The machine was several hundred thousand £s worth of lasers, optical table and stepper motors moving a stage that must have w

I had a wonderful Heisenbug once. The machine was several hundred thousand £s worth of lasers, optical table and stepper motors moving a stage that must have weighed half a ton. The purpose was to print digital holograms, line by line, on glass plates. Between each line there was a delay of some seconds, during which time various calculations took place; even the smallest 3"x3" holograms took half an hour to print. Larger ones could run overnight. Once printed, the (expensive) plates needed another half an hour of darkroom wet processing, after which they could be briefly viewed (the image disappeared again until the plate was completely dry - uneven shrinking destroys the interference pattern). The byzantine hardware control software had been written long before my time - my job was merely to write the imaging software that fed this machine as it ran. There was an elaborate startup procedure to follow, and just before you'd hit "start" you'd run my little C program, which would synchronise with a timing signal from a serial port. Then, as the stage whirred back and forth, it would feed images to a little display - 30 Hz, pixel by pixel, line by line. One day, just as we were getting confident of the rhythm of operating the machine, a larger hologram came out "back to front" in a strange way - one of the parallax axes was reversed. Perplexed, we carefully tried again with the same file - and it came out fine the second time. Unable to reproduce the issue, the matter was dropped. Some weeks later, just as our confidence had begun to wax again - boom. Another inverted hologram. Another large plate wasted. Another careful investigation, another failure to reproduce. A Heisenbug! This continued happening, off and on, most vexingly - and of course, never when I was there, and only with the larger, more expensive plates. Eventually one day I came in and it had happened again, and this time I noticed something peculiar - the hologram had finished printing, but my little C program was just sitting there, waiting on seemingly the last line, for a timing signal that would never come. And the light began to dawn. My C program had the basic structure "buffer line, wait for signal, display images, go to start". It turned out that what was happening was people were hitting "start" too soon after launching it, before it had had a chance to properly buffer the first line. It thus missed the "start line" signal of the first line - which caused it to print line 1 as line 2, line 2 as line 3, etc. And because it was a rastering stage, this meant that every line was printed in the opposite direction to what was expected. The really treacherous thing about it was, it only manifested when I wasn't there because I knew instinctively to allow the program enough time to buffer. It only happened with big, expensive plates because they had longer lines which took longer to buffer. And it only happened at all when the operator of the machine was sufficiently confident, or in a hurry, to perform the final two tasks of the checklist within seconds of each other. The fix? A note in the checklist. I miss the job, but I don't miss debugging that darned machine.