In the last batch of edits, a change was made that created a new translation unit 151, but something has gone wrong: The new unit appears as untranslated in the Special:Translate page, but clicking its link has the translation window waiting indefinitely for the text, and when I export the translations in Gettext format, the unit is simply missing.
It's hard to guess what went wrong, so I've simply deleted the tag and a new one, unit 152, is now created and is working correctly.
Thanks. Fortunately I saved my translation locally so it is just a matter of copy and paste to complete the translation.
Usually you can pick up discarded translations through the history, but I guess that in this case it wouldn't actually be saved anywhere other than your local copy. The browser back-button sometimes gives a local cache that can be copied, otherwise there's really no answer to this.
The main problems seem to occur when new contributors manually add translation tags (despite there being warnings all over the place against this). I couldn't see any evidence that this was the cause of this specific one, though.