The following position was reached in the first round of the ACT Championships. It is Black's move and the first decision is whether to go into a king and pawn ending with Rg1+. To make the correct call, Black needs to be able to calculate whether the ending is winning/drawn/lost. In positions like this counting tempo correctly is crucial.
Black decided to play 43. ... Rg1+ and after the forced 44.Qxg1 Rxg1+ 45.Kxg1 Kxd5 46.Kf2 he then had to decide how to clean up the a and b pawns. Meanwhile the spectators (including me) had come to the conclusion that Black now wins by a tempo with 46. ... Kc4 47.Kf3 Kb3 48.Kf4 Kxb2 Turns out we were horribly wrong as White is winning by a tempo in this position. However Black was a little smarter than we were as he grabbed the a pawn with 48. ... Kxa4 (instead of 48. ... Kxb2) and after 49.Kxf5 Kb3 50.Kg5 Kxb2 51.Kxh5 a5 52.Kg6 a4 53.h5 a3 54.h6 a2 55.h7 a1(Q) 56.h8(Q)+ Kb1 57.Qxa1+ the game was drawn.
However, the post mortem revealed how tricky these endings really are, as White missed a win with 48.a5! forcing Black to take an extra move to capture the a pawn, and losing by a tempo. And if 48.a5! was good for White, it turned out that 46. ... a5! was just as good for Black, when Black ignores the queenside pawns and instead shepherds the f pawn up the board instead!