I use a wiki server similar to commons which transcludes images to other servers. And I got the following error when trying to edit a page concerning an image description:
Exception from line 1475 of /var/www/html/mediawiki/phase3/includes/GlobalFunctions.php: wfTimestamp() called with illegal output type.

Looking at getTimestamp() called in File.php, I found that the first argument is missing. Thus I added 0:
function getTimestamp() {

		$path = $this->getPath();
		if ( !file_exists( $path ) ) {
			return false;
		}
		return wfTimestamp(0, filemtime( $path ) );

}
And it works now but I am surprised by this bug and my response is only a workaround.

Version: 1.12.x
Severity: critical