One of 'em happens when the Gekko slams down foot and it gets stuck. Press circle in front of it and you leap on head and stab repeatedly. While back think people were talking about one that happens when you're close to a wall maybe? Never has happened with me.
The fire of justice is burning bright and we are the flames. Believe in the Shield.
There's a low chance that if you execute an enemy near a wall you get another type of execution; to prevent you from clipping through the walls or creating other strange bugs. It looks a bit like Skyhigh (>>S) and kills everything it connects with instantly.