What are the Major mistakes that one will do in GDs/PI??

According to me using slang words like yaar, princi, and other short forms leaves a bad impression about you in the first place, you'll be talking like as if to your friends and it'll give impression that you are not serious enough about what is going on, and they will think that you are not professional enough to be in the management with that attitude,  and then more over GDs are about giving quality content to the topic and not some slangs which will iritate the panel...So remember guys, don't use slangs in an GD/PI

Until and unless you are from some other country where you have difficulty in speaking English or there is totally different accent, you don't do that, it will give impression that the person is showing off and is trying to be more visible in the group as such, and this will not go well with the panel which is judging you, be simple, speak normal English, gel with the group and try to be more visible with your points that you are making and not with some different accent, which will do no good to you....