What is it? *You don't have to say every sport*
I just had some free time and felt like typing my opinion:
In basketball-being rejected by the rim is the most embarrassing thing you can do as a basketball player in the NBA. You're paid millions of dollars to do anything and you can't make the easiest shot of them all. (That, missing free throws or airballing a shot)
In football-being a quarterback and pulling your arm forward to pass, and letting the ball slip out behind your back. Granted, it rarely happens, but he looks like a darned fool when his primary job is to throw the ball forward and it goes behind his head and he's looking all around for it and by the time he finds the fumble, he is being tackled while the defensive tackle is returning the ball to the endzone. :P
In baseball-a guy looks like a complete idiot when he throws the ball into the stands, thinking it's the 3rd out, when it really was only the 1st or 2nd...It happened this past year to somebody and he was all smiles and laughing at the fans as he threw the ball up there and then turned around toward his teammates and was like,"What? What do you mean--Oh ****!" or something like that...You could tell he was surprised/embarrassed to find out. It's embarrassing for the guy when he shows the whole world that he didn't even know how many outs there were.
In hockey-getting beat down in a fist-fight when you were the guy that started it has got to be the most embarrassing. You thought you could take him and you end up getting your face smashed in in front of millions of viewers around the country. Plus, you've got to go have stitches in your face. Feel the pain!