Bootstrap 3.1.0 was released today: http://blog.getbootstrap.com/2014/01/30/bootstrap-3-1-0-released/

The current version of lessphp can't compile this because of "extend", here is the error"

lessphp fatal error: parse error: failed at "&:extend(.clearfix all);" bootstrap-less/mixins.less on line 648

This is related to issue #498 on supporting "Extend". It looks like this is a big issue now.