Comment 12 for bug 741385

I can confirm the issue exists, but I'm not certain it's all that clear that the issue definitely is in libappindicator; downgrading ubuntu-mono 0.0.24 has the icon cache properly generate, which is not the case with ubuntu-mono 0.0.25 and 0.0.26.

I think the error messages that come from configuring ubuntu-mono are the key to this and point to the exact cause: there's something wrong in an icon or the directory structure that yields the cache's hashes to be incorrectly calculated:

gtk-update-icon-cache: The generated cache was invalid.
WARNING: icon cache generation failed for /usr/share/icons/ubuntu-mono-dark
gtk-update-icon-cache: The generated cache was invalid.
WARNING: icon cache generation failed for /usr/share/icons/ubuntu-mono-light