Failing Retaining Wall

You noticed it on a walk around the yard: a retaining wall that is leaning a little too far, bulging in the middle, cracked, or quietly shedding soil after every rain. Now you can't stop looking at it. Is it about to come down? Will fixing it cost a few hundred dollars or tens of thousands? And if it sits on the line with your neighbor, whose problem is it anyway? You posted a photo online and got...