Some years ago, I was assisting at an Organ recital on new year's eve at my city's main church, the last piece was supposed to be Elgar's pomp & circumstance march no.1, arranged for two organs.
We programmed all the combinations for that one and all the other pieces on the day before(there were lots and lots).
Fast forward to the end of the recital, I take away the sheet music for the penultimate piece and activate the first combination for the march. The organist raises his hands and starts playing -- and all we hear is a principal 8' only, completely wrong sound. Somehow, all prepared combinations had disappeared! He managed to perform the piece somehow, but the recital's supposed climax ended a bit underwhelming.
Sadly, we never found out, why the combinations vanished (most likely, they were saved to another bank and we didn't notice, or somebody else switched banks/erased them accidentally, before the recital started).
Nerve-wracking indeed :(