When PHP is generated with random content (e.g. #1239), the post-processor cache gets disabled as of #1325 to prevent polluting the object cache with garbage cache values. This only matters when a persistent object cache is present, since that is when the post-processor cache is used.

When the style sanitizer comes across a stylesheet, it parses it and stores the cached result in a transient. This is done regardless of whether there is a persistent object cache present, since parsing CSS is expensive. If, however, the CSS has variable content in any way then this can result in a site with out a persistent object cache to have its wp_options table balloon to hundreds of thousands of garbage transients, as seen on this support topic.

I believe such variable stylesheets to be an uncommon occurrence, but when it does happen, we should warn the user and stop storing the parsed CSS in transients. There could also be a parameter to the style sanitizer to disable the transient cache, as well as provide a threshold for what constitutes an unacceptable rate of cache misses.