Add test and doc

mne/io/cnt/cnt.py

@@ -336,6 +336,7 @@ def _get_cnt_info(input_fname, eog, ecg, emg, misc, data_format, date_format, he
                 )
                 n_bytes = 2
                 n_samples = data_size // (n_bytes * n_channels)
+                # See: PR #12393
                 annotations = _read_annotations_cnt(input_fname, data_format="int16")
                 if annotations.onset[-1] * sfreq > n_samples:
                     n_bytes = 4

mne/io/cnt/tests/test_cnt.py

@@ -16,6 +16,7 @@
 
 data_path = testing.data_path(download=False)
 fname = data_path / "CNT" / "scan41_short.cnt"
+# Contains bad spans and could not be read properly before PR #12393
 fname_bad_spans = data_path / "CNT" / "test_CNT_events_mne_JWoess_clipped.cnt"
 
 
@@ -86,6 +87,16 @@ def test_compare_events_and_annotations():
     assert "STI 014" not in raw.info["ch_names"]
 
 
+@testing.requires_testing_data
+def verify_reading_bytes():
+    raw_16 = read_raw_cnt(fname, preload=True)
+    raw_32 = read_raw_cnt(fname_bad_spans, preload=True)
+
+    # Verify that the number of bytes read is correct
+    assert len(raw_16) == 3070
+    assert len(raw_32) == 90000
+
+
 @testing.requires_testing_data
 def test_bad_spans():
     """Test reading raw cnt files with bad spans."""