diff --git a/src/wp-includes/html-api/class-wp-html-processor.php b/src/wp-includes/html-api/class-wp-html-processor.php
index 6e1723494c2e9..b5a20dbd82cdc 100644
--- a/src/wp-includes/html-api/class-wp-html-processor.php
+++ b/src/wp-includes/html-api/class-wp-html-processor.php
@@ -432,6 +432,11 @@ public function next_tag( $query = null ) {
 	 * @return bool Whether a tag was matched.
 	 */
 	public function step( $node_to_process = self::PROCESS_NEXT_NODE ) {
+		// Refuse to proceed if there was a previous error.
+		if ( null !== $this->last_error ) {
+			return false;
+		}
+
 		if ( self::PROCESS_NEXT_NODE === $node_to_process ) {
 			$top_node = $this->state->stack_of_open_elements->current_node();
 			if ( $top_node && self::is_void( $top_node->node_name ) ) {
@@ -744,6 +749,10 @@ private function bookmark_tag() {
 	 * @return string|null Name of currently matched tag in input HTML, or `null` if none found.
 	 */
 	public function get_tag() {
+		if ( null !== $this->last_error ) {
+			return null;
+		}
+
 		$tag_name = parent::get_tag();
 
 		switch ( $tag_name ) {
diff --git a/tests/phpunit/tests/html-api/wpHtmlProcessor.php b/tests/phpunit/tests/html-api/wpHtmlProcessor.php
index 6a666c19cf27c..d6b818dd44c34 100644
--- a/tests/phpunit/tests/html-api/wpHtmlProcessor.php
+++ b/tests/phpunit/tests/html-api/wpHtmlProcessor.php
@@ -42,6 +42,40 @@ public function test_warns_that_the_static_creator_methods_should_be_called_inst
 		);
 	}
 
+	/**
+	 * Once stepping to the end of the document, WP_HTML_Processor::get_tag
+	 * should no longer report a tag. It should report `null` because there
+	 * is no tag matched or open.
+	 *
+	 * @ticket 59167
+	 *
+	 * @covers WP_HTML_Processor::get_tag
+	 */
+	public function test_get_tag_is_null_once_document_is_finished() {
+		$p = WP_HTML_Processor::createFragment( '<div class="test">Test</div>' );
+		$p->next_tag();
+		$this->assertSame( 'DIV', $p->get_tag() );
+
+		$this->assertFalse( $p->next_tag() );
+		$this->assertNull( $p->get_tag() );
+	}
+
+	/**
+	 * Ensures that if the HTML Processor encounters inputs that it can't properly handle,
+	 * that it stops processing the rest of the document. This prevents data corruption.
+	 *
+	 * @ticket 59167
+	 *
+	 * @covers WP_HTML_Processor::next_tag
+	 */
+	public function test_stops_processing_after_unsupported_elements() {
+		$p = WP_HTML_Processor::createFragment( '<p><x-not-supported></p><p></p>' );
+		$p->next_tag( 'P' );
+		$this->assertFalse( $p->next_tag(), 'Stepped into a tag after encountering X-NOT-SUPPORTED element when it should have aborted.' );
+		$this->assertNull( $p->get_tag(), "Should have aborted processing, but still reported tag {$p->get_tag()} after properly failing to step into tag." );
+		$this->assertFalse( $p->next_tag( 'P' ), 'Stepped into normal P element after X-NOT-SUPPORTED element when it should have aborted.' );
+	}
+
 	/**
 	 * Ensures that the HTML Processor maintains its internal state through seek calls.
 	 *