We ran into this problem on the Hydra Wiki Platform since ResourceLoader shares CSS caches across wikis where it can.(Which is great!) There is the ability to set Less variables for the wiki's skin CSS through the administrator control panel. However, while ResourceLoader would generate the proper CSS the compileLessFile() function in FileModule would get a blank set of variables for hashing into the cache key since it was not pulling from the context. This resulting in wikis fighting each other trying to rebuild slightly different CSS on each page load.
Note: I did not write this patch. Another member of the team here wrote it.