twbs/bootstrap — pull request #14281 by aentan (2014-07-29 16:32:27+00:00)
Title: Fixes double border in justified button groups.

Uses `:before` to create a 1px-wide strip of same background color as the button to cover the extra border. Modification added to `.button-variant` mixin to handle hover/focus/active states, i.e. match the colors of other states.

Not really in the same spirit as negative margins but definitely closer to it than nuking borders.