Steps to reproduce: Search for something in the Commons search. Try to step through the search result pages. Many thumbnails need to be recreated because nobody requested that thumbnail size for the last few weeks and it's not in a cache because of this. All these thumbnails will fail to load.
What "fail" means: The search result page displays but the thumbnails do not. The HTTP requests for the thumbnails never return. My browser (I'm using Opera) waits a minute but stops because nothing happened.
When I try to reload the pages the thumbnails are displayed sometimes.
When I step to the next search result page the same happens again. Loading the thumbnails needs two minutes or simply fails.