'Nor let us commit fornication, as some of them committed, and fell in one day three and twenty thousand.'
Here the sin of sexual immorality is more clearly spelled out. If the reference is to Numbers 25:1-3 it also includes being influenced by idolatry, and eating in the presence of idols. But 'us' shows his consciousness that the sin is one he too might commit, so he does not emphasise the connection with idols directly. Sexual impropriety was highly prevalent in Corinthian society, as it is for many today. It was so easy to think, 'everyone does it, it is part of modern culture'. But Paul condemns it out of hand.
There would appear to have been a possible tradition that 23,000 died 'in one day', with the remainder dying soon after, making 24,000 in all (Numbers 25:9), or it may be that Paul is accentuating the severity of the punishment by stressing how quickly the large majority died, while not wishing to commit himself to all dying in one day.
The 24,000 may well have deliberately reflected twice twelve stressing the intensity of the punishment on the twelve tribes. Paul would recognise this. His 23,000 would then reflect the large majority, but not all, as dying in one day by a simple reduction by a fraction (a thousand). Numbers were regularly used in this kind of way in those days, to convey ideas rather than be exact. Note the mention of 'the day of the plague' in Numbers 25:18 which draws attention to the severity of the first day.
It is extremely unlikely that Paul got it wrong accidentally. He knew the Scriptures too well. That Paul clearly saw the 'one day' as significant in expressing the severity of the punishment comes out in the next verse where the imperfect suggests that in contrast the snake judgment occurred over a period of time, but he was clearly wary of saying that all without exception died in one day, thus he reduced the number lightly.