diff --git a/src/wp-includes/class-wp-block-processor.php b/src/wp-includes/class-wp-block-processor.php
index 7c22dc1ea87cd..b97d21633adff 100644
--- a/src/wp-includes/class-wp-block-processor.php
+++ b/src/wp-includes/class-wp-block-processor.php
@@ -831,6 +831,11 @@ public function next_token(): bool {
 					$this->open_blocks_at[]         = $after_prev_delimiter;
 					$this->open_blocks_length[]     = 0;
 					$this->was_void                 = true;
+
+					if ( $backup > 0 ) {
+						$this->last_error = self::INCOMPLETE_INPUT;
+					}
+
 					return true;
 				}
 
diff --git a/tests/phpunit/tests/block-processor/wpBlockProcessor.php b/tests/phpunit/tests/block-processor/wpBlockProcessor.php
index 6f8934e003030..7bc507c705245 100644
--- a/tests/phpunit/tests/block-processor/wpBlockProcessor.php
+++ b/tests/phpunit/tests/block-processor/wpBlockProcessor.php
@@ -362,6 +362,66 @@ public static function data_incomplete_html_comments_that_are_not_delimiters() {
 		);
 	}
 
+	/**
+	 * Verifies that trailing bytes which could start a block comment delimiter
+	 * are not reported as a delimiter.
+	 *
+	 * @ticket 66138
+	 *
+	 * @dataProvider data_documents_ending_in_a_partial_delimiter
+	 *
+	 * @covers ::next_token()
+	 *
+	 * @param string      $html        Input document.
+	 * @param string[]    $block_types Printable block type of every delimiter in the document, in order.
+	 * @param string|null $last_error  Expected error after scanning the entire document.
+	 */
+	public function test_reports_no_delimiter_for_partial_delimiter_at_end_of_document( $html, $block_types, $last_error ): void {
+		$processor = new WP_Block_Processor( $html );
+
+		$found = array();
+		while ( $processor->next_delimiter() ) {
+			$found[] = $processor->get_printable_block_type();
+		}
+
+		$this->assertSame(
+			$block_types,
+			$found,
+			'Should have found only the delimiters which are in the document.'
+		);
+
+		$this->assertSame(
+			$last_error,
+			$processor->get_last_error(),
+			'Should have reported the expected error after reaching the end of the document.'
+		);
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array<string, array{0: string, 1: string[], 2: string|null}>
+	 */
+	public static function data_documents_ending_in_a_partial_delimiter(): array {
+		return array(
+			// Documents ending in a partial delimiter with no earlier delimiter.
+			'Ends in <'                   => array( 'text<', array(), WP_Block_Processor::INCOMPLETE_INPUT ),
+			'Ends in <!'                  => array( 'text<!', array(), WP_Block_Processor::INCOMPLETE_INPUT ),
+			'Ends in <!-'                 => array( 'text<!-', array(), WP_Block_Processor::INCOMPLETE_INPUT ),
+
+			// Documents ending in a partial delimiter after an earlier delimiter.
+			'Opener, then ends in <'      => array( '<!-- wp:a -->text<', array( 'core/a' ), WP_Block_Processor::INCOMPLETE_INPUT ),
+			'Opener, then ends in <!'     => array( '<!-- wp:a -->text<!', array( 'core/a' ), WP_Block_Processor::INCOMPLETE_INPUT ),
+			'Opener, then ends in <!-'    => array( '<!-- wp:a -->text<!-', array( 'core/a' ), WP_Block_Processor::INCOMPLETE_INPUT ),
+			'Void, then ends in <!-'      => array( '<!-- wp:a /-->text<!-', array( 'core/a' ), WP_Block_Processor::INCOMPLETE_INPUT ),
+			'Closer, then ends in <'      => array( '<!-- /wp:a -->text<', array( 'core/a' ), WP_Block_Processor::INCOMPLETE_INPUT ),
+
+			// Documents which do not end in a partial delimiter.
+			'Contains < but ends in text' => array( 'a<b', array(), null ),
+			'Opener and trailing text'    => array( '<!-- wp:a -->text', array( 'core/a' ), null ),
+		);
+	}
+
 	/**
 	 * Verifies that block delimiters are matched even with malformed
 	 * JSON attributes as long as they start and end with curly brackets.