In chapter 8 of his epistle, the author of Hebrews attributes the following words to God:
Hebrews 8:9
9 Not according to the covenant that I made with their fathers in the day when I took them by the hand to lead them out of the land of Egypt; because they did not continue in My covenant, and I disregarded them, says the Lord.
This is a direct quotation from the book of Jeremiah. But, in the book of Jeremiah, the passage actually says:
Jeremiah 31:32
32 Not according to the covenant that I made with their fathers in the day that I took them by the hand to lead them out of the land of Egypt, My covenant which they broke, though I was a husband to them, says the Lord.
This discrepancy is due to the author of Hebrews quoting a Greek translation of the book of Jeremiah. The Hebrew Masoretic Text, which is favored by almost every single English translation in this passage, reads as presented above - "though I was a husband to them", or similar.
The source of the Greek reading is thought to be the change of a single Hebrew letter at the start of the phrase - "ג" instead of "ב". Note the issues that come about from "God" allegedly inspiring a text in a language which is imprecise enough to allow one letter being changed to make such a tremendous difference in what is being said, and also doing the "inspiration" before the invention of robust copying technology.
So, the question for Christians would be - what did God say? The author of Hebrews says that God said the Greek textual variant. The Old Testament in almost every Bible says that God did not say that, but instead said something completely different.
Note that the issue is not the "overall tenor of the book of Jeremiah", or something to that effect. A direct quote is being made. And it is being attributed to God. God either said one, or the other. In either case, almost every Bible would have an error in either Jeremiah, or Hebrews, and would be falsely putting words into God's mouth, which He did not say.
Further reading - The author of Hebrews presents a similar dilemma with his quotation of Psalm 40:6 in Hebrews 10:5, in which the readings are very different