Boils. After the frogs, "he hardened his heart" (Exodus 8:15), and after the flies, "Pharaoh hardened his heart this time also" (Exodus 8:32). At the boils the wording turns around: "the Lord hardened the heart of Pharaoh" (Exodus 9:12). The Lord does it again at the locusts and the darkness (Exodus 10:20; Exodus 10:27).
Whether Pharaoh or the Lord did the hardening, the text twice adds that it happened just as the Lord had said (Exodus 8:15; Exodus 9:12). A note in the New Oxford Annotated Bible says the back-and-forth probably comes from older stories that were later combined. In the finished text, the note adds, it may also show the pull between God's plan and Pharaoh's own will.
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