Charles Ellicott Commentary 2 Kings 4:40

"So they poured out for the men to eat. And it came to pass, as they were eating of the pottage, that they cried out, and said, O man of God, there is death in the pot. And they could not eat thereof." — 2 Kings 4:40 (ASV)

There is death in the pot. —The bitter taste, and perhaps incipient effect of the stew, made them think of poison.