Beagle 2 was a British-led lander that hitched a ride on the european space agency's Mars Express mission, meant to touch down on mars on christmas day 2003. it went silent after separating from the main spacecraft and was presumed to have crashed or failed completely. the actual answer only came in 2015, when NASA's Mars Reconnaissance Orbiter spotted it in high resolution images, it had actually landed mostly intact, but some of its solar panels failed to fully unfold, and that blocked the antenna it needed to communicate back to earth.
so close to working the whole time and nobody knew for over a decade, that's such a rough near miss
makes you wonder how many other failed missions were actually this close to functioning