I was nugetting down dotLess and Twitter Bootstrap Less to play around with only to run into problems. 

The error I got was:

Syntax Error on line 1394

http://localhost:1337/Content/less/bootstrap.less on line 1394, column 3:

On line 1394, there's this code:

filter: e(%("progid:DXImageTransform.Microsoft.gradient(startColorstr='%d', endColorstr='%d', GradientType=1)",@startColor,@endColor)); // IE9 and down

As I understand it there are differences how the implementations of less parsers work. dotLess doesn't handle the above string but I guess the Ruby folks have no problem with it. The simple solution is to add qualifiers to the string, like this:

filter: e(%("\"progid:DXImageTransform.Microsoft.gradient(startColorstr='%d', endColorstr='%d', GradientType=1)\"",@startColor,@endColor)); // IE9 and down

Adding the two escaped quotes solved the problem.