I first read about it on Critical Section and thought that it's a neat concept:

A HeisenBug is a bug whose presence is affected by [the] act of observing it.

Well, I finally got mine! I administer several Mac OS X Server machines. And of course I make regular, i.e. daily backups. To an external firewire harddrive, to be precise. For this I use a shareware called Tri-Backup. It's a cool tool and does what it does well. However, being a GUI-Application, it needs to be running in order to perform automated backups. Which in turn mandates, that a user with sufficient privileges be logged in. Also, it means, that in order to access its Error and History Reports, one must regularily access the server physically. Which I do every now and then.

Of course, each time I do that, I expect everything to be all right. So imagine my shock sometime last year, when upon unlocking the screen saver of a particular machine I was greeted with an error message:

"Oh no! No device, no backup! For how many days?!" Instant relief, though: all backups up until then had been carried out successfully. So I remounted the harddrive and came back next day. Same message! But: the backup had been performed, again. This time I waited a few days. Same thing. And now, yesterday, after the holidays I made a final check. And again!

So what seems to happen is, that the process of unlocking the screen (to check on the backup) somehow makes the Volume unmount. If I don't check: everything goes fine...

Oh, well, I've been meaning to switch to RsyncX anyway...