Interesting
A project is moving from their initial layouts to Bootstrap. They have quite a few class name conflicts in their css, so in order to isolate the new Bootstrap styles to specific elements, they added the Bootstrap styles to their own (in SASS) under a .bootstrap namespace, and are nesting any new elements under divs with a class of .bootstrap, and now everything is pretty again.
Nesting in this way (rather than namespacing all of the preexisting css) ensures that no Bootstrap styling leaks into pre-existing elements, but means that occasionally a pre-Bootstrap style will leak into the new namespaced elements due to higher selector specificity.