A number of times, I have seen new translations popping up for stuff that had already been translated, sometimes simply wrong, sometimes better than the original. Often this happened after some database problem (when the yellow markers pop up again for a short while).
A good idea to avoid to have to go over translations over and over again for quality control**, is to first look at the history of the particular snippet (there is a link next to the snippet on the translation page, saying "x changes" or something --cannot open translation pages right now). If you see yellow untranslated text, and there are changes for that snippet, you can roll back to one of the former translations via that feature.
**Yes. If we don't want to look like Flickr, we should do it. Word-by-word translations will aways look bad.
