1. Freezing at the beginning of the game.
Many people have a problem with getting the objective after they kill hand to hand the guy, that is 50 metres to the market place where you chase down the guy which is sent by Kobin.
You need to kill that guy BY CROUCHING and if you freeze again, try executing the other 2 guys in front of you.
2. You make a hand to hand kill, but you can't mark enemies.
This is another bug, which happens more often.
If you can't Mark & Execute BUT you have made a hand to hand kill you will need to do another one!
3. You cannot shoot while you are grabbed to a wall/pipe.
In this case you will need to do the only fixing thing that I discovored.
You will need to go back on the ground, that's the disicion.
4. You cannot mark & execute Reed's men at the end of the game (at the White House).
This bug happens very very often.
In this case, you'll need to restart from last checkpoint.
BTW, if you know other bugs and how to fix them, please write to the discussion.
If I have time, I'll post and screenshots!