Back in the Atari ST days, it was quite popular for people to make "bootsector" demos, showcasing some sort of graphical effect shoehorned into the first sector of a floppy disk (512 bytes) so that the effect would be shown when you booted from that floppy.
As you might imagine, these demos became quite elaborate over time, and it wasn't long before people started making bootsector demos that could remove all the borders on the ST (also called "full overscan"), as well as showing a nice graphical effect. Removing the borders on an ST involved exploiting a bug in the video chip and some extremely careful timing, so doing this and showing a nice graphical effect, all in 512 bytes (less, if you count the bits of the bootsector you couldn't change) was all the rage at one point.
Of course, I had to get in on the action, so I made my own "full overscan" bootsector demo, and was so proud of it that I put in on quite a few of the floppies I used regularly, and also gave it to some friends who did the same.
Then, one day, my friend phoned me and said the demo had gone a bit wonky on one of his disks (it's a bit hard to describe -- it looked like the demo, but somehow not). During the course of trying to find out what was wrong with it, he tried another disk, and it was doing the same thing. And another. And another -- all the disks he had with the bootsector on them, in fact.
So I tried my disks. Same problem. On all of them. After much messing around, getting nowhere, we removed the bootsector from most (but not all) disks, and I spent the next few days trying to find out what was wrong with it.
After a few days, I got it working again. Now, this was before I'd ever heard of things like source control, so I wasn't sure what I'd changed to fix it in those few days, just that I somehow had. Remember, this overscan effect required perfect timing, so making demos like this work involved a lot of semi-mindless fiddling with the code and instruction timings, etc.
So I phoned my friend and told him I'd fixed it. While we were talking, I tried the disk just to double check it was ok, which it was. Until I realised: this was a different disk, one I hadn't "fixed".
Not quite sure what was going on, my friend tried one of his remaining disks with the old code on it. It worked.
As it turns out, the new code was the same as the old code. My days of fiddling with instruction timings had led me back to where I started, but yet it was now working.
All the disks we had that contained the bootsector were now working again.
They continued working for some time, maybe a week or so, and then, one day, all of them stopped working again. And then they started working again, and so on. At any given time, they either all worked, or none of them did.
We never did find out why this was happening. We eventually put it down to some periodic effect at the hardware level that affected how the overscan trick behaved. We were never 100% convinced by this explanation, but it was more comforting than the prospect of all instances of the bootsector being entangled in some spooky, quantum way, which was our next best guess. :)