The Beagle II, the 2003 UK Mars probe that failed. We knew it had got down and at least partially deployed but some new imaging techniques have revealed that just the last solar panel failed to deploy and prevented the transmitter from operating. It might have been carrying out it's observations and experiments for years. It still might be carrying them out, but we can't hear it. All that way and just one final component refused to work.
You really do feel for the scientists involved in this. One poor guy even had the same Martian atmospheric sensor he had on Beagle put on the Schiaparelli lander, and look what happened to that.
Oh well, Mars ain't going anywhere...