Sit & Go Mistakes
We all make mistakes. The key is to being a money winning poker player is to minimize your mistakes. I cant promise you that these are most common mistakes made in Sit & Go tournaments, but they are made and they cost money in the long run.
Playing to many hands
Playing in tournaments you cant afford to get broke. The reason is of course that you just cant put in more money on the table and continue playing. When your chips are gone you are out. Therefore you need to play good starting hands and stay away from speculative plays.
Calling raises with to week hands
If someone raised a pot you need a good hand to call that raise. If you just call a raise with a hand that is ok, but not great you will find yourself in a though decision after the flop. And that is something you should avoid! AA, KK, QQ are great hands and also AK. The two first one you cant usually lay down. But hands below that you might call with if the raise is small. And if you play your AA, KK so make sure to reraise. And why not go all in!?
Not changing gear
In Sit & Go tournament poker its important to be able to change gears. I believe that if you are in a normal Sit & Go you should try to get ITM and then you can speed up and gamble to try to take first place. But even in a earlier in a Sit & Go you can notice that people are just playing to tight. Then change gears and try to pick up some pots from position 8,9 and the button. I don't think you should play worthless hands, but you can lover your starting hands requirements a bit.