I was having a problem trying to use “less” (lesscss v1.2.0) with Twitter Bootstrap (v1.4).
I could see LESS pull all the bootstrap *.less files off the server but the CSS would render properly. In many cases it would have the error:
Cannot call method 'slice' of undefined
The reason is a bug in the most recent version of less.js. To resolve the problem I downgraded to “less” v1.1.5