A trial in Liverpool for attempted robbery collapsed because a Muslim witness had sworn on the Bible, not the Koran.
“You have heard evidence from the witness on oath, just like you all swore an oath,” the judge told the jury. “Unfortunately it was sworn on the Bible and he just informed us he is a Muslim, which means I can’t accept his evidence and neither can you, because it was sworn on the wrong holy book. We have to start the case again.”
The next day, the judge thought better of it and stated in court that “in hindsight, it is a personal matter for any witness as to how they wish to be sworn”. It was a bit late, as the trial had been stopped. But the incident does show that swearing an oath is not just a quaint custom.