diff --git a/aioesphomeapi/connection.py b/aioesphomeapi/connection.py
index 4245c7c..232550c 100644
--- a/aioesphomeapi/connection.py
+++ b/aioesphomeapi/connection.py
@@ -537,25 +537,34 @@ class APIConnection:
             # If the task was cancelled, we need to clean up the connection
             # and raise the CancelledError as APIConnectionError
             self._cleanup()
-            if not isinstance(ex, APIConnectionError):
-                cause: Exception | None = None
-                if isinstance(ex, CancelledError):
-                    err_str = "Starting connection cancelled"
-                    if self._fatal_exception:
-                        err_str += f" due to fatal exception: {self._fatal_exception}"
-                        cause = self._fatal_exception
-                else:
-                    err_str = str(ex) or type(ex).__name__
-                new_exc = APIConnectionError(
-                    f"Error while starting connection: {err_str}"
-                )
-                new_exc.__cause__ = cause or ex
-                raise new_exc
-            raise ex
+            raise self._wrap_fatal_connection_exception("starting", ex)
         finally:
             self._start_connect_task = None
         self._set_connection_state(ConnectionState.SOCKET_OPENED)
 
+    def _wrap_fatal_connection_exception(
+        self, action: str, ex: BaseException
+    ) -> APIConnectionError:
+        """Ensure a fatal exception is wrapped as as an APIConnectionError."""
+        if isinstance(ex, APIConnectionError):
+            return ex
+        cause: BaseException | None = None
+        if isinstance(ex, CancelledError):
+            err_str = f"{action.title()} connection cancelled"
+            if self._fatal_exception:
+                err_str += f" due to fatal exception: {self._fatal_exception}"
+                cause = self._fatal_exception
+        else:
+            err_str = str(ex) or type(ex).__name__
+            cause = ex
+        if isinstance(self._fatal_exception, APIConnectionError):
+            klass = type(self._fatal_exception)
+        else:
+            klass = APIConnectionError
+        new_exc = klass(f"Error while {action} connection: {err_str}")
+        new_exc.__cause__ = cause or ex
+        return new_exc
+
     async def _do_finish_connect(self, login: bool) -> None:
         """Finish the connection process."""
         in_do_connect.set(True)
@@ -585,22 +594,7 @@ class APIConnection:
             # If the task was cancelled, we need to clean up the connection
             # and raise the CancelledError as APIConnectionError
             self._cleanup()
-            if not isinstance(ex, APIConnectionError):
-                cause: Exception | None = None
-                if isinstance(ex, CancelledError):
-                    err_str = "Finishing connection cancelled"
-                    if self._fatal_exception:
-                        err_str += f" due to fatal exception: {self._fatal_exception}"
-                        cause = self._fatal_exception
-                else:
-                    err_str = str(ex) or type(ex).__name__
-                    cause = ex
-                new_exc = APIConnectionError(
-                    f"Error while finishing connection: {err_str}"
-                )
-                new_exc.__cause__ = cause or ex
-                raise new_exc
-            raise ex
+            raise self._wrap_fatal_connection_exception("finishing", ex)
         finally:
             self._finish_connect_task = None
         self._set_connection_state(ConnectionState.CONNECTED)
diff --git a/tests/common.py b/tests/common.py
index 19855ff..362f14c 100644
--- a/tests/common.py
+++ b/tests/common.py
@@ -117,3 +117,14 @@ def send_plaintext_connect_response(
 def send_ping_response(protocol: APIPlaintextFrameHelper) -> None:
     ping_response: message.Message = PingResponse()
     protocol.data_received(generate_plaintext_packet(ping_response))
+
+
+def get_mock_protocol(conn: APIConnection):
+    protocol = APIPlaintextFrameHelper(
+        connection=conn,
+        client_info="mock",
+        log_name="mock_device",
+    )
+    transport = MagicMock()
+    protocol.connection_made(transport)
+    return protocol
diff --git a/tests/test__frame_helper.py b/tests/test__frame_helper.py
index ae838fc..b7ce35b 100644
--- a/tests/test__frame_helper.py
+++ b/tests/test__frame_helper.py
@@ -4,7 +4,7 @@ import asyncio
 import base64
 from datetime import timedelta
 from typing import Any
-from unittest.mock import MagicMock
+from unittest.mock import MagicMock, patch
 
 import pytest
 from noise.connection import NoiseConnection  # type: ignore[import-untyped]
@@ -20,16 +20,17 @@ from aioesphomeapi._frame_helper.plain_text import (
     _cached_varuint_to_bytes as cached_varuint_to_bytes,
 )
 from aioesphomeapi._frame_helper.plain_text import _varuint_to_bytes as varuint_to_bytes
+from aioesphomeapi.connection import ConnectionState
 from aioesphomeapi.core import (
+    APIConnectionError,
     BadNameAPIError,
     HandshakeAPIError,
     InvalidEncryptionKeyAPIError,
     ProtocolAPIError,
-    SocketAPIError,
     SocketClosedAPIError,
 )
 
-from .common import async_fire_time_changed, utcnow
+from .common import async_fire_time_changed, get_mock_protocol, utcnow
 
 PREAMBLE = b"\x00"
 
@@ -385,6 +386,10 @@ def test_bytes_to_varuint(val, encoded):
     assert cached_bytes_to_varuint(encoded) == val
 
 
+def test_bytes_to_varuint_invalid():
+    assert bytes_to_varuint(b"\xFF") is None
+
+
 @pytest.mark.asyncio
 async def test_noise_frame_helper_handshake_failure():
     """Test the noise frame helper handshake failure."""
@@ -568,3 +573,52 @@ async def test_noise_frame_helper_handshake_success_with_single_packet():
 
     with pytest.raises(ProtocolAPIError, match="Connection closed"):
         helper.data_received(encrypted_header + encrypted_payload)
+
+
+@pytest.mark.asyncio
+async def test_init_plaintext_with_wrong_preamble(conn: APIConnection):
+    loop = asyncio.get_event_loop()
+    protocol = get_mock_protocol(conn)
+    with patch.object(loop, "create_connection") as create_connection:
+        create_connection.return_value = (MagicMock(), protocol)
+
+        conn._socket = MagicMock()
+        await conn._connect_init_frame_helper()
+        loop.call_soon(conn._frame_helper._ready_future.set_result, None)
+        conn.connection_state = ConnectionState.CONNECTED
+
+    task = asyncio.create_task(conn._connect_hello_login(login=True))
+    await asyncio.sleep(0)
+    # The preamble should be \x00 but we send \x09
+    protocol.data_received(b"\x09\x00\x00")
+
+    with pytest.raises(ProtocolAPIError):
+        await task
+
+
+@pytest.mark.asyncio
+async def test_eof_received_closes_connection(
+    plaintext_connect_task_with_login: tuple[
+        APIConnection, asyncio.Transport, APIPlaintextFrameHelper, asyncio.Task
+    ],
+) -> None:
+    conn, transport, protocol, connect_task = plaintext_connect_task_with_login
+    assert protocol.eof_received() is False
+    assert conn.is_connected is False
+    with pytest.raises(SocketClosedAPIError, match="EOF received"):
+        await connect_task
+
+
+@pytest.mark.asyncio
+async def test_connection_lost_closes_connection_and_logs(
+    caplog: pytest.LogCaptureFixture,
+    plaintext_connect_task_with_login: tuple[
+        APIConnection, asyncio.Transport, APIPlaintextFrameHelper, asyncio.Task
+    ],
+) -> None:
+    conn, transport, protocol, connect_task = plaintext_connect_task_with_login
+    protocol.connection_lost(OSError("original message"))
+    assert conn.is_connected is False
+    assert "original message" in caplog.text
+    with pytest.raises(APIConnectionError, match="original message"):
+        await connect_task
diff --git a/tests/test_client.py b/tests/test_client.py
index d50b001..00a7e1c 100644
--- a/tests/test_client.py
+++ b/tests/test_client.py
@@ -58,12 +58,7 @@ from aioesphomeapi.model import (
 )
 from aioesphomeapi.reconnect_logic import ReconnectLogic, ReconnectLogicState
 
-from .common import (
-    PROTO_TO_MESSAGE_TYPE,
-    Estr,
-    generate_plaintext_packet,
-    get_mock_zeroconf,
-)
+from .common import Estr, generate_plaintext_packet, get_mock_zeroconf
 
 
 @pytest.fixture
diff --git a/tests/test_connection.py b/tests/test_connection.py
index f38b8c5..6fe816e 100644
--- a/tests/test_connection.py
+++ b/tests/test_connection.py
@@ -31,6 +31,7 @@ from .common import (
     async_fire_time_changed,
     connect,
     generate_plaintext_packet,
+    get_mock_protocol,
     send_ping_response,
     send_plaintext_connect_response,
     send_plaintext_hello,
@@ -41,17 +42,6 @@ from .conftest import KEEP_ALIVE_INTERVAL
 KEEP_ALIVE_TIMEOUT_RATIO = 4.5
 
 
-def _get_mock_protocol(conn: APIConnection):
-    protocol = APIPlaintextFrameHelper(
-        connection=conn,
-        client_info="mock",
-        log_name="mock_device",
-    )
-    transport = MagicMock()
-    protocol.connection_made(transport)
-    return protocol
-
-
 @pytest.mark.asyncio
 async def test_connect(
     plaintext_connect_task_no_login: tuple[
@@ -152,7 +142,7 @@ async def test_disconnect_when_not_fully_connected(
 @pytest.mark.asyncio
 async def test_requires_encryption_propagates(conn: APIConnection):
     loop = asyncio.get_event_loop()
-    protocol = _get_mock_protocol(conn)
+    protocol = get_mock_protocol(conn)
     with patch.object(loop, "create_connection") as create_connection:
         create_connection.return_value = (MagicMock(), protocol)
 
@@ -357,7 +347,7 @@ async def test_plaintext_connection_fails_handshake(
     """
     loop = asyncio.get_event_loop()
     exception, raised_exception = exception_map
-    protocol = _get_mock_protocol(conn)
+    protocol = get_mock_protocol(conn)
     messages = []
     protocol: APIPlaintextFrameHelper | None = None
     transport = MagicMock()
diff --git a/tests/test_log_runner.py b/tests/test_log_runner.py
index aec65ca..e393675 100644
--- a/tests/test_log_runner.py
+++ b/tests/test_log_runner.py
@@ -14,7 +14,6 @@ from aioesphomeapi.connection import APIConnection
 from aioesphomeapi.log_runner import async_run
 
 from .common import (
-    PROTO_TO_MESSAGE_TYPE,
     Estr,
     generate_plaintext_packet,
     get_mock_async_zeroconf,