What have I done?!
Compare that to:
You'll see that there's a slight difference.
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The same problem had plagued me since my upgrade to 13.04 until I finally found this post:
A bug in pango-graphite is behind this.
The following fixed it for me.
$ sudo apt-get purge libpango1.0-common pango-graphite
If you don't have libpango1.0-common installed it can be omitted from the command.
$ sudo apt-get purge pango-graphite