Numbers 25:9
9 And those that died in the plague were twenty and four thousand.
1 Corinthians 10:8
8 Neither let us commit fornication, as some of them committed, and fell in one day three and twenty thousand.
This is a well-known alleged contradiction. Did 23,000 or 24,000 people die due to fornication?
Firstly, we can recognize that if 24,000 died, it's also true that 23,000 died, and one account is just being less precise.
However, read carefully - Paul is careful to specify that 23,000 fell in one day. That leaves 1,000 that died later.
Are we grasping at straws, though? How could Paul have known this? Because before he became a Christian, he was a ruler of the pharisees (Philippians 3:5), meaning that he no doubt knew a lot of things about Jewish history that weren't explicity in Scripture.
He demonstrates this later when he tells us that Adam was not deceived, but took the fruit from Eve with full knowledge of what he was doing (1 Timothy 2:14) - details that we are not given in Genesis 3.
Similarly, he tells us in Hebrews 11:19 that Abraham knew that if God allowed him to sacrifice Isaac, that God would raise him from the dead, and that it would be a figure of what was to come - details we are not given in Genesis 22.
Also in Hebrews, he talks about something Moses said, of which there is no record of in the Old Testament at all (Hebrews 12:21). He does the same thing with a quotation from Christ in Acts 20:35, which isn't in any of the four Gospels.
Another instance of this kind of thing in Scripture is Jude 1:9, where Jude gives a detail of what happened after Deuteronomy 34, which isn't recorded or mentioned anywhere else in either Testament.
A pharisee was someone who dedicated their entire life to obeying the Law and knowing the religious history of the Jews, so him knowing this kind of trivia shouldn't be too surprising. In Israel during the time of Jesus, they doubtless had dozens of books filled with facts or traditions about the Old Testament that God didn't see fit to preserve for whatever reason - probably much of it was hearsay.
In summary, don't treat the "in one day" to be incidental. It would certainly be impossible to prove that this contradicts, given that precise wording being used.