So doing a mistake in video game is defined as doing an action and getting unwanted outcome for it, the simplest example that comes to my mind is Geometry Dash tap the screen in the wrong moment and you lose, simple right? you get it? Now a more complex example can be playing as ADC in League of Legends (if you haven't played a LoL ranked never in your life you won't understand this example), when you're ADC there is a lot of times that you'll die for getting in the wrong position in the wrong time but this is really hard to define because due the amounts of variables that can happen in LoL a wrong position varies depending on you team composition on enemies one, what are your items, which adc are you playing, who's your support... The game has so many variables that it makes it deep, complex and refreshing...
But anyway returning to the topic what is a mistake in MTA? because depending of the game and the amount of variable within it can be harder to define and realize when you made one that ended up stepping you into the death. I'll try to give some examples in order to make you understand.
-Be too passive only playing reactively, basically waiting your rival to do the first move and react to it rather than trying to trick them
-Too agressive, staying all the time shooting and never defending yourself not getting cover
-Standing still while getting shooted by more players than you
-Swapping to the wrong weapon depending the situation (ex: not swapping from Spaz to Uzi in order to sprint in the case that sneak not existing)
Then depending on the certain situation a mistake can vary alot because there are moments where you want to be extremely agressive or very passive that's where the game complexity is and that's where a part of your skill has to shine.