When running build themes task:

php admin/cli/build_theme_css.php --themes=yourtheme -v

If you have a in your css a line that reads as follows:

border-radius: 100px / 10px;

The task will fail. Since this is an adhoc task that is called by the upgrade routine, if the task fails, then the cron can end abnormally and prevent tasks after the task from being run.  This is traced down lib/rtlcss/RTLCSS.php around line 268 - getListComponents. If the values are separated by spaces instead of slashes the build themes task will complete.

This has the potential to disrupt the performance of any site using a theme that uses the border-radius CSS setting.