If you use Visual Studio (2015, in my case) to create an ASP.NET Webforms Web Application, with Bootstrap theming, by default the glyphicons are stored in the /fonts folder off the project root.
If you start rearranging your bootstrap files under /Content into subfolders (to support multiple themes, for example), all the references within bootstrap to ../fonts/
will break. You can either move the fonts folder to the appropriate relative path under /Content
, or else rename all the references from ../fonts
to /fonts
.