Only?
What about that 2003 playoffs game 1, round 1 Spurs vs Phoenix when the Spurs were up by two, 5.1sec left in the game TIM DUNCAN missed a crucial free throw, then out of nowhere Marbury decided to take a hero - ball made a 3ptr .... Phoenix won the game.
I should have used blue font. My reply was sarcastic as if Leonard has been the only player to miss critical fts.
Leonard's only 23. I see no reason why he couldn't improve his ft shooting.

And 2003, maybe the best year of Duncan's career but we already know what happened after that, they bounched back BIG TIME and won their 2nd le.
Now, Duncan has improved his free throw a lot better, learned from it.
