When viewing Special:newfiles on commons, lots of thumbs don't render. Looking at the requests, the affected files all have same error message:
<html><head><title>Error generating thumbnail</title></head>
<h1>Error generating thumbnail</h1>
Error creating thumbnail:
<!-- http://commons.wikimedia.org/w/thumb_handler.php/e/e0/2004-12-01_500-Millibar_Height_Contour_Map_NOAA.png/120px-2004-12-01_500-Millibar_Height_Contour_Map_NOAA.png -->
<!-- mw1153 -->
When visiting the url a second time, the file renders (and is cached). I would guess the request is being routed to a different backend server, and there's just something wrong with mw1153.