If a user goes to a page on our site and adds a .html extension, they receive our correct 404 page...but there is also the following php warning that is output to the page body:
Warning: is_dir() [function.is-dir]: open_basedir restriction in effect. File(...wp-content/cache/supercache/site.org/3096/post_that_exists/index.html/) is not within the allowed path(s): (/nfs:/tmp:/usr/local:/etc/apache2/gs-bin) in .../wp-content/plugins/wp-super-cache/wp-cache-phase1.php on line 558
I'm running the latest version of your plugin and WordPress (it is running in Multisite mode).
Let me know if I can provide any more information. Thanks!