I remember witnessing Virgil flub a piece to the point he had to start over. But instead of admitting his mistake, he quickly took out his handkerchief and wiped the keys down as if to blame the error on the sweat drips on the keys.
yes, every organist will for various reasons have a bad day at some point in their playing year. Sudden nervousness, fingers that feel and move like they're made of lead, and sudden onset amnesia are common problems.
Just as public speakers have to imagine the audience lightly clothed, nervous organists should imagine the congregation being completely tone-deaf. Whatever mistakes you make do not matter, as long as there's some remnants of a melody in there somewhere you'll be fine.
On one occasion I fumbled my way through the intro of every hymn in a service, but played the verses fine.
I've had some.
1. I was just finishing the prelude for Mass when the power to the organ died. It made this awful groaning sound as the last air faded through the pipes and then it was dead. Fortunately it happened before Mass and not during.
2. There was a time when I was missing the last page of music and I didn't realize it until I was already playing. Thankfully, memory kicked at the right time and I managed to go through without the music, but it could have been a major disaster considering I'm usually NOT good at playing by memory.
Those are disasters/near disasters that didn't involve my playing. If we want to count the latter, then I'll just say that I've had plenty more!