diff --git a/test-all-native.sh b/test-all-native.sh
index dfb9eec..6206704 100755
--- a/test-all-native.sh
+++ b/test-all-native.sh
@@ -567,6 +567,8 @@ grep -F 'tests/gcode/linuxcnc_close_and_downcase_words.ngc' test-linuxcnc-source
 grep -F 'tests/gcode/linuxcnc_percent_file_ignores_trailing.ngc' test-linuxcnc-source-link.sh >/dev/null
 grep -F 'linuxcnc_unclosed_comment_error|Unclosed comment found' test-linuxcnc-source-link.sh >/dev/null
 grep -F 'linuxcnc_nested_comment_error|Nested comment found' test-linuxcnc-source-link.sh >/dev/null
+grep -F 'linuxcnc_commented_percent_error|Bad character '\''%'\'' used' test-linuxcnc-source-link.sh >/dev/null
+grep -F 'linuxcnc_mid_program_percent_error|Bad character '\''%'\'' used' test-linuxcnc-source-link.sh >/dev/null
 
 for script in \
     test-native.sh \
diff --git a/test-linuxcnc-source-link.sh b/test-linuxcnc-source-link.sh
index 241d439..4543e68 100755
--- a/test-linuxcnc-source-link.sh
+++ b/test-linuxcnc-source-link.sh
@@ -1022,9 +1022,13 @@ done
 # read_one_item() then uses the default reader table from interp_array.cc
 # and rejects unknown item-start characters, including middle-of-line or
 # comment-prefixed N/O words, midline O-words, bracket expressions, naked numbers, and
-# non-leading ATAN argument slashes.
+# non-leading ATAN argument slashes. LinuxCNC src/emc/rs274ngc/rs274ngc_pre.cc
+# open() only enables percent-file mode when the first nonblank line contains
+# only '%', so commented or non-opening percent lines reach read_one_item().
 bad_character_error_cases=(
     "linuxcnc_bad_character_bang_error|Bad character '!' used"
+    "linuxcnc_commented_percent_error|Bad character '%' used"
+    "linuxcnc_mid_program_percent_error|Bad character '%' used"
     "linuxcnc_bad_character_middle_n_word_error|Bad character 'n' used"
     "linuxcnc_comment_prefixed_n_word_error|Bad character 'n' used"
     "linuxcnc_comment_prefixed_spaced_n_word_error|Bad character 'n' used"
diff --git a/tests/gcode/linuxcnc_commented_percent_error.ngc b/tests/gcode/linuxcnc_commented_percent_error.ngc
new file mode 100644
index 0000000..2a838e6
--- /dev/null
+++ b/tests/gcode/linuxcnc_commented_percent_error.ngc
@@ -0,0 +1,3 @@
+% (not a delimiter)
+G0 X1
+%
diff --git a/tests/gcode/linuxcnc_mid_program_percent_error.ngc b/tests/gcode/linuxcnc_mid_program_percent_error.ngc
new file mode 100644
index 0000000..19388c8
--- /dev/null
+++ b/tests/gcode/linuxcnc_mid_program_percent_error.ngc
@@ -0,0 +1,3 @@
+G0 X1
+%
+G0 Y2