Strip the newlines off of the XML before the Warn(). Also, we should never pass user-supplied data as a format string.

stepmania/src/Background.cpp

@@ -723,7 +723,9 @@ void BackgroundImpl::Layer::UpdateCurBGChange( const Song *pSong, float fLastMus
 		if( iter == m_BGAnimations.end() )
 		{
 			XNode *pNode = change.m_def.CreateNode();
-			LOG->Warn( "Tried to switch to a background that was never loaded\n" + XmlFileUtil::GetXML(pNode) );
+			RString xml = XmlFileUtil::GetXML( pNode );
+			Trim( xml );
+			LOG->Warn( "Tried to switch to a background that was never loaded:\n%s", xml.c_str() );
 			SAFE_DELETE( pNode );
 			return;
 		}