It was noticed in a downstream bug comment[1] that some navigational links on the top of the Yelp window do not appear translated, despite being translated in the po file. I saw quite a few of these with gnome-user-docs 3.2.1, which was entirely translated to Hungarian. These include: 

Desktop Help, Hardware, Hardware problems, Settings, Sound,
Photos, Videos, Music and players: http://people.ubuntu.com/~kelemeng/pix/breadcrumb.png etc.

The common thing about these is that they have an msgctxt context in the po file, but this is not being taken into account when merging.

For example, on the mouse page we can see the untranslated Settings and Hardware categories, but the rest of the navigation looks to be alright, translation is merged and displayed: http://people.ubuntu.com/~kelemeng/pix/mouse-tips.png
If I change the translations slightly, it becomes obvious which instance is used:
http://people.ubuntu.com/~kelemeng/pix/mousetips-plain.png

So the one with the context marker is ignored, and this leads to untranslated entries, because in many cases there are no msgctx-less instances of the affected strings.

[1]: https://bugs.launchpad.net/ubuntu-docs/+bug/814822/comments/3