Kludex
diff --git a/python_multipart/multipart.py b/python_multipart/multipart.py
@@ -1361,18 +1361,19 @@ def data_callback(name: CallbackName, end_i: int, remaining: bool = False) -> No
                         # We matched the whole boundary string.
                         index = boundary_length - 1
                         i = i0 + boundary_length - 1
+                        c = data[i]
                     else:
-                        # No match found for whole string.
-                        # There may be a partial boundary at the end of the
-                        # data, which the find will not match.
-                        # Since the length to be searched is limited to the
-                        # boundary length, scan the tail for boundary[0] via
-                        # bytes.find (C-level) to keep cost off the Python loop.
-                        i = max(i, length - boundary_length)
-                        j = data.find(boundary[:1], i, length - 1)
-                        i = j if j >= 0 else length - 1
-
-                    c = data[i]
+                        # No whole boundary, but the tail may hold a partial one
+                        # that completes in the next chunk. Boundary starts with
+                        # CR, which an RFC boundary contains nowhere else, so the
+                        # last CR in the tail is the only candidate prefix start.
+                        k = data.rfind(boundary[:1], max(i, length - boundary_length + 1), length)
+                        if k != -1 and boundary.startswith(data[k:length]):
+                            index = length - k
+                        # Carry the partial via index; the end-of-chunk flush
+                        # emits the data before it and re-marks the lookbehind.
+                        i = length
+                        continue
 
                 # Now, we have a couple of cases here.  If our index is before
                 # the end of the boundary...

diff --git a/tests/test_data/http/crlf_dense_part_data.http b/tests/test_data/http/crlf_dense_part_data.http
@@ -0,0 +1,34 @@
+--boundary
+Content-Disposition: form-data; name="file"; filename="crlf.bin"
+Content-Type: application/octet-stream
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+--boundar
+
+
+
+
+
+--bound
+
+--boundary--
diff --git a/tests/test_data/http/crlf_dense_part_data.yaml b/tests/test_data/http/crlf_dense_part_data.yaml
@@ -0,0 +1,8 @@
+boundary: boundary
+expected:
+  - name: file
+    type: file
+    file_name: crlf.bin
+    content_type: 'application/octet-stream'
+    data: !!binary |
+      DQoNCg0KDQoNCg0KDQoNCg0KDQoNCg0KDQoNCg0KDQoNCg0KDQoNCg0KLS1ib3VuZGFyDQoNCg0KDQoNCg0KLS1ib3VuZA0K
diff --git a/tests/test_multipart.py b/tests/test_multipart.py
@@ -779,6 +779,7 @@ def test_not_aligned(self) -> None:
     "almost_match_boundary_without_LF",
     "almost_match_boundary_without_final_hyphen",
     "single_field_single_file",
+    "crlf_dense_part_data",
 ]
 
 EPILOGUE_TEST_HEAD = (