Im no expert btw :)

A mistake is when you lose some form of advantage, don't capitalize on a possible advantage or you get fragged. Most typical mistakes are choosing the wrong path or switching to the wrong weapon in battle.

"How do you analyse demos the best?"

Firstly as a new player you should be watching other players more than you watch yourself. As a new player you're making so many big and small mistakes that demo analyzing would be tedious and probably a waste of time. For a while just simply playing and watching people should start to stamp out some of your flaws by trial and error. eg1. If you switch to rail when your opponent is running towards you, you'll probably get fragged or receive a lot more damage than you deliver. On that note when you watch players try to emulate them, so be able to move around the map like them for the most part; if they are jumping and gaining speed and your not maybe you should work on your strafejumping.

Now to answer the question somewhat, when you analyze a demo firstly don't watch it like t.v(starring at the screen to get entertained). You generally want to have a purpose and questions before even watching a demo and the simpler you keep your purpose/questions the more likely you'll learn something. The most basic question's to ask is:
- Why did I die here
- How could I have avoided this fight
- Could I avoided getting spawn killed
- How could I defend myself here

For me I only watch a demo when I feel I had the advantage at some point but somehow lost it. GLHF :)

in theory everything is a mistake where you could have done better... that goes for decision making, aim, positioning, timing, metagame etc... obv theres some more crucial than others.

as for demo viewing, in game a lot of going on, our thoughts being occupied with a lot of things + emotions come into play too. however if you sit down with paying attention and cant spot any of your mistakes than youre a delusional egomaniac. it comes naturally as you watch yourself as a 3rd person you surely will find a lot of spot where ull be just like "aha.. i could have done this instead of that and it would be so much better".

althought its only useful if you analyze close matches or those where you was somewhat of a vital opponent. analyzing how strenx raped you is no use, cos he just does orbits better in every aspect of the game, so even if youd put up your very best performance it would be futile.

dont analise... just find the style that suits you the most, and start playing... doesnt matter if u lose at start, the aim is to make YOUR game flawless...

ofc i dont talk about things like dropping from ztn RA to mh when enemy got lg out and similar stuff :{D