pipeline =
  Babel.begin()
  |> Babel.fetch(["some", "nested", "path"])
  |> Babel.map(Babel.into(%{atom_key: Babel.fetch("string-key")}))

data = %{
  "some" => %{
    "nested" => %{
      "path" => [
        %{"unexpected-key" => :value1},
        %{"unexpected-key" => :value2},
        %{"unexpected-key" => :value3}
      ]
    }
  }
}

Babel.apply!(pipeline, data)

Which will produce the following error

** (Babel.Error) Failed to transform data: [not_found: "string-key", not_found: "string-key", not_found: "string-key"]

Root Cause(s):
1. Babel.Trace<ERROR>{
  data = %{"unexpected-key" => :value1}

  Babel.fetch("string-key")
  |=> {:error, {:not_found, "string-key"}}
}
2. Babel.Trace<ERROR>{
  data = %{"unexpected-key" => :value2}

  Babel.fetch("string-key")
  |=> {:error, {:not_found, "string-key"}}
}
3. Babel.Trace<ERROR>{
  data = %{"unexpected-key" => :value3}

  Babel.fetch("string-key")
  |=> {:error, {:not_found, "string-key"}}
}

Full Trace:
Babel.Trace<ERROR>{
  data = %{"some" => %{"nested" => %{"path" => [%{"unexpected-key" => :value1}, %{"unexpected-key" => :value2}, %{"unexpected-key" => :value3}]}}}

  Babel.Pipeline<>
  |
  | Babel.fetch(["some", "nested", "path"])
  | |=< %{"some" => %{"nested" => %{"path" => [%{"unexpected-key" => :value1}, %{...}, ...]}}}
  | |=> [%{"unexpected-key" => :value1}, %{"unexpected-key" => :value2}, %{"unexpected-key" => :value3}]
  |
  | Babel.map(Babel.into(%{atom_key: Babel.fetch("string-key")}))
  | |=< [%{"unexpected-key" => :value1}, %{"unexpected-key" => :value2}, %{"unexpected-key" => :value3}]
  | |
  | | Babel.into(%{atom_key: Babel.fetch("string-key")})
  | | |=< %{"unexpected-key" => :value1}
  | | |
  | | | Babel.fetch("string-key")
  | | | |=< %{"unexpected-key" => :value1}
  | | | |=> {:error, {:not_found, "string-key"}}
  | | |
  | | |=> {:error, [not_found: "string-key"]}
  | |
  | | Babel.into(%{atom_key: Babel.fetch("string-key")})
  | | |=< %{"unexpected-key" => :value2}
  | | |
  | | | Babel.fetch("string-key")
  | | | |=< %{"unexpected-key" => :value2}
  | | | |=> {:error, {:not_found, "string-key"}}
  | | |
  | | |=> {:error, [not_found: "string-key"]}
  | |
  | | Babel.into(%{atom_key: Babel.fetch("string-key")})
  | | |=< %{"unexpected-key" => :value3}
  | | |
  | | | Babel.fetch("string-key")
  | | | |=< %{"unexpected-key" => :value3}
  | | | |=> {:error, {:not_found, "string-key"}}
  | | |
  | | |=> {:error, [not_found: "string-key"]}
  | |
  | |=> {:error, [not_found: "string-key", not_found: "string-key", not_found: "string-key"]}
  |
  |=> {:error, [not_found: "string-key", not_found: "string-key", not_found: "string-key"]}
}