Sass uses a cache system (stored in the invisible .sass-cache folder) to speed up the creation of CSS. Very occasionally, something goes wrong in Sass and Compass land. Maybe the CSS won't generate when the Sass files are saved, or perhaps the wrong styles seem to be getting generated. If things are going pear shaped and it's not an error between the keyboard and chair, it's possible to force a clear of the Sass cache using a Compass command.

It's no exaggeration to say I've used this command less than a handful of times. However, on the off chance you need it, from the CLI, move to the root of the relevant project and run this command

compass clean