Everybody makes mistakes

...especially when the clock is ticking. The reigning Women's World Champion, Anna Ushenina, joins the large and distinguished group led by Grandmaster Vladimir Epishin who have failed to mate in fifty moves with king, bishop, and knight against king.  It's really not that hard if you've studied the ending. (I think I could do it on increment alone, but I'm honestly not sure: I saw games between strong class players in the 2012 U.S. Open and a recent Chicago Open in which the game was similarly drawn.)

Ushenina knew the "W" technique for driving the defending king to a corner controlled by the bishop. But she didn't understand the one position in which the defender seems to escape the side of the board, only to be caught in a net that includes the "danger corner" controlled by the bishop.

Suppose you want to drive the defending king from a1 (a dark corner) to h1 (a light corner controlled by your bishop). 

• You will want to put your knight on c2 to keep the defender from returning to a1. (Ushenina did this this on move 78.) 
• Then you will want to put your bishop on a2 (star move!) to keep the defender from returning to b1. (Ushenina did this on move 79.) 
• Now the horse on c2 is freed to execute the "W-manuever" to transfer the knight from c2 to e2. (A knight always controls squares of the opposite color, so you want to hurry your horse from c2 to e2 as quickly as possible: it's not doing you any good when it's controlling squares of the same color as the bishop.) 
• Ushenina's first major inexactitude was 82.Bd5?! The correct move here is 82.Ne2, and if Black tries to break toward freedom with 82...Kf3, then 83.Be6! Now White controls e4 with king, f4 and g3 with knight, and g4 and h3 with bishop: that's a very effective force field!
• Olga Girya ran away from the dangerous h1 square to the relative safety of the h8 square. 
• On move 101, Ushenina seems to have lost confidence in her technique, as she fails to implement the winning 101.Bh7! (an idea she'd already used earlier.) 

I was an expert for many years before I learned the correct technique from a beginner's book: thank you, Pandolfini's Endgame Course.