That's not in my Bible
Many Christians are not familiar with large sections of the Bible; and, as such, have not have noticed some of its odder or more abhorrent passages, such as the injunction against wearing clothes of wool and linen (Deuteronomy 22:11 ) or Jephthah's human sacrifice (Judges 11:30-40 ). Such people may express disbelief that such things are in the Bible.
The best way to counter this argument is to show the person the relevant passage. There are several web sites and books that list questionable passages in the Bible.
There are also searchable Bible applications for smartphones, PDAs, and other mobile devices. It can be useful to have one as a reference.
The most famous incident of human sacrifice is the Akedah,(Genesis 22:1-24 ) the Binding of Isaac. It is worth noting that, though Abraham is commanded by God to sacrifice Isaac at the mountaintop, God intervenes at the last moment. There has been a great deal of work on this in Jewish apologetics and Christian apologetics. There is a corresponding story in Islam, which credits the son as Ismail instead of Isaac.
Jephthah ritually sacrifices his daughter to God (Judges 11:29-40 ) after vowing to sacrifice the first thing that greets him when he returns from his military victory over the Ammonites.
After the Israelites violate God's commandments, God commands Joshua to instruct the people in ritual repentance, which includes the destruction of those who violated the covenant by fire. (Joshua 7:15 ) It is ambiguous as to whether this is ritual sacrifice performed by the tribe or whether it is performed directly by God.