Regarding first, the Decalogue, what I was surprised to learn in the story of Exodus, was that the Israelites time at Mt. Sinai did not sync up with when the commandments were written. Most specifically, the 10th commandment, "Thou shall not covet your neighbor's house; your neighbor's wife, or male or female slave, or ox or donkey or anything that belongs to your neighbor." They are in the wilderness, what do they own? They don't own houses out in the wilderness. I had never put that together.
The part of the Book of the Covenant that surprised me was in Exodus 21:22. I knew an "eye for eye..." was in the Old Testament; Jesus quotes this verse, but I did not realize it was part of a specific sin example. The example, when a pregnant woman is accidently injured and loses the baby.
Although, I knew that there was a story about a scapegoat in the Old Testament, and in general what it was about (see ESPN's Films: Catching Fire, inside the Bartman debacle), I didn't realize that they were sending the goat out to appease either another god (Azazel), an "angry" or "fierce god" - demonic figure (NOAB), or possibly the angry side of God (Kevin Smith's take).