diff --git a/crates/clawhdf5-format/src/object_header.rs b/crates/clawhdf5-format/src/object_header.rs
index b28fdad..2c3c819 100644
--- a/crates/clawhdf5-format/src/object_header.rs
+++ b/crates/clawhdf5-format/src/object_header.rs
@@ -421,6 +421,13 @@ impl ObjectHeader {
             };
             pos += msg_header_size;
 
+            // `end` is where the messages stop and the checksum starts.
+            // libhdf5 bounds a message by the chunk including its checksum,
+            // but a message that runs into the checksum still fails there:
+            // its loop stops at the checksum, and reading the checksum from
+            // past its start overruns the chunk ("ran off end of input
+            // buffer while decoding"). Both refuse it; only the text
+            // differs.
             if msg_data_size > end - pos {
                 return Err(FormatError::InvalidObjectHeader(
                     "message size exceeds buffer end",
diff --git a/crates/clawhdf5/tests/header_validation_interop.rs b/crates/clawhdf5/tests/header_validation_interop.rs
index 7cdd4b4..2f52d27 100644
--- a/crates/clawhdf5/tests/header_validation_interop.rs
+++ b/crates/clawhdf5/tests/header_validation_interop.rs
@@ -252,6 +252,54 @@ for libver in ("earliest", "latest"):
     }
 }
 
+/// A v2 object header message whose size runs past the messages into the
+/// chunk's checksum is refused by libhdf5 whether it runs 1 byte or more
+/// into the checksum (its loop stops at the checksum, and then the checksum
+/// read and the size check fail), so it is refused here too.
+#[test]
+fn v2_header_message_running_into_the_checksum_is_refused() {
+    skip_if_no_python!();
+    let dir = tempfile::tempdir().unwrap();
+    let body = format!(
+        "{FIX_OHDR_PY}{}",
+        r#"
+good = os.path.join(d, "good.h5")
+with h5py.File(good, "w", libver="latest") as f:
+    f.create_dataset("d", data=np.arange(4, dtype="<i4"))
+raw = bytearray(open(good, "rb").read())
+off = raw.rfind(b"OHDR")
+flags = raw[off + 5]; p = off + 6
+if flags & 0x20: p += 16
+if flags & 0x10: p += 4
+width = 1 << (flags & 3)
+end = p + width + int.from_bytes(raw[p:p + width], "little"); p += width
+hdr = 6 if flags & 0x04 else 4
+last = p
+while p + hdr <= end:
+    last = p
+    p += hdr + int.from_bytes(raw[p + 1:p + 3], "little")
+assert p == end
+size = int.from_bytes(raw[last + 1:last + 3], "little")
+for k in (1, 4, 5):
+    bad = bytearray(raw)
+    bad[last + 1:last + 3] = (size + k).to_bytes(2, "little")
+    fix_ohdr(bad, off)
+    open(os.path.join(d, f"into_checksum_{k}.h5"), "wb").write(bad)
+"#
+    );
+    let verdicts = h5py_verdicts(dir.path(), &body);
+    assert_agrees_with_h5py(
+        dir.path(),
+        &verdicts,
+        &[
+            "good ok",
+            "into_checksum_1 ERROR",
+            "into_checksum_4 ERROR",
+            "into_checksum_5 ERROR",
+        ],
+    );
+}
+
 #[test]
 fn truncated_files_are_refused_and_nothing_past_the_end_of_file_is_read() {
     skip_if_no_python!();