For those who use custom/downloaded version of Bootstrap 3:
A couple of weeks ago Bootstrap 3.1 was released, so default documentation on getbootstrap.com covers the new version only.
There were some breaking changes.
Yii2 and some extensions still use 3.0.
So if you’re wondering why your modals/whatever stopped working, check your extensions or syntax.
Thanks for the tip. 
My biggest issue was that you couldn’t use multiple visible-XX classes, eg:
<div class="visible-xs visible-sm">…
You could use combined hidden- classes though.
They actually fixed this issue in the recently released 3.1.1 though, but I’d already gone through my entire site changing everything from visible- to hidden-. It was a bit of a pain.