This is Paul writing in 1 Corinthians 10:8:
Almost everyone seems to agree that this is a reference to Numbers 25:9:
I can think of a number of ways that an infallibilist could reply:
There's a lot of wriggle room here, though none of these answers are particularly convincing. Eschewing these possibilities, the NIV Study Bible says instead:
This would be a good argument if Paul had changed the number to a nice round sum, such as 25,000. But he didn't. By far the most likely explanation is that Paul simply made an mistake - he was probably writing from memory, or he might have had a bad translation of Numbers. He certainly would have written 24,000 if he had known that was what the Hebrew said.
Still, if you think that Paul simply didn't care about the precise number, you can say that this is not really an error. But it feels like an error.
Updated: 2008-05-29
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