Update TemplateFile class with a clearAll() static method and update ProcessPageView to use it when a 404 is thrown. This ensures that a 404 thrown after some output has already been sent can't get interpreted as a non-404 by the client

wire/core/TemplateFile.php

@@ -138,7 +138,15 @@ class TemplateFile extends WireData {
 	 * DEPRECATED: Variables that will be applied globally to this and all other TemplateFile instances
 	 *
 	 */
-	static protected $globals = array(); 
+	static protected $globals = array();
+
+	/**
+	 * Output buffer starting level, set by first TemplateFile instance that gets created
+	 * 
+	 * @var null|int
+	 * 
+	 */
+	static protected $obStartLevel = null;
 
 	/**
 	 * Construct the template file
@@ -147,6 +155,7 @@ class TemplateFile extends WireData {
 	 *
 	 */
 	public function __construct($filename = '') {
+		if(self::$obStartLevel === null) self::$obStartLevel = ob_get_level();
 		if($filename) $this->setFilename($filename); 
 	}
 
@@ -561,6 +570,24 @@ class TemplateFile extends WireData {
 		return self::$renderStack;
 	}
 
+	/**
+	 * Clear out all pending output buffers
+	 * 
+	 * @since 3.0.175
+	 * @return int Number of output buffers cleaned
+	 * 
+	 */
+	public static function clearAll() {
+		$n = 0;
+		if(self::$obStartLevel !== null) {
+			while(ob_get_level() > self::$obStartLevel) {
+				ob_end_clean();
+				$n++;
+			}
+		}
+		return $n;
+	}
+
 	/**
 	 * The string value of a TemplateFile is its PHP template filename OR its class name if no filename is set
 	 * 

wire/modules/Process/ProcessPageView.module

@@ -227,6 +227,7 @@ class ProcessPageView extends Process {
 
 		} catch(Wire404Exception $e) {
 			// 404 exception
+			TemplateFile::clearAll();
 			return $this->renderNoPage(array(
 				'reason404' => '404 thrown during page render',
 				'exception404' => $e,