The Famous Reit study. It even has it's own Wiki entry
http://en.wikipedia.org/wiki/R%C3%A9ti_endgame_study
A wonderful teaching example with so few pieces on the board.
If you cannot pick up anything from the study then give up.
Apparently the idea of the study came from Lasker - Tarrasch 1914.
(OK here we go again. Greenpawn endgame analysis.) 😳
Miscalculation by my opponent going into a won ending once won me a game.
G.Chandler - I.Ferguson, Edinburgh C.C. v Edinburgh University 1981.
i was staring at the posted puzzle and only kept on thinking, because it was so obvious a loss for white, that it had to be a draw - otherwise no puzzle. still i didnt solve it.
once i read a different puzzle: in which square is the diagonal as long as the sides? before you start thinking of squares painted on weird 3d objects... it is the chess board, of course.
it is intuition to treat the chess diagonale as longer then its sides, hard to get rid off in chess.
good to know a caliber of tarrasch fell for it, too...