Am using Twenty Eleven theme with WP 3.3. Whenever I edit the theme with the in-built editor of WP, I get an Internal Server Error (The server encountered an internal error or misconfiguration and was unable to complete your request...) upon saving the file. The problem doesn't arise when working with other themes.
This is kind of intriguing since Twenty Eleven is a WordPress developed theme and there can't be anything wrong with it. At the same time the problem doesn't occur with other themes.