Actually this thread just made me think of a new guy we had last week. He came in and made some very crucial mistakes which led to a very uncomfortable day 2.
#1- On his first day I asked the teacher a question during technique and he tried to answer it. Definitely don't answer any BJJ related technical questions on your first day.
#2- When rolling with him he was throwing up rubber guard like he was Eddie Bravo. Stick to the basics when you start. Fuck the shit you see on youtube.
#3- After getting submitted by one of our white belts he asked him "How much do you weigh?" Once he found out he was outweighed by 30 lbs. he said "Oh, well that is why then." Don't be the "Whatcha weigh guy?", it just makes you look bad.
#4- He was a complete spaz.
#5 - When the teacher was going over some basic armbars from when the guy tries to choke you inside your guard he proclaimed that I had done that to him. Referring to chokin him inside his guard. This was a problem because I stopped doing that years ago. In fact it's one of the things my instructor jokes with me about, how I used to try and gi choke everyone inside their guard. To proclaim that in front of people makes me look like I don't know what I'm doing and worse I don't listen to my instructor.
The next day me and the white belt he had questioned about his weight to were training open mat and he showed up. I decided to smash him. I was drilling extra hard (including Margarida pass from combat base while he wasn't wearing a cup. Ouch.). And I just smashed him during sparring. 100 Kilos!! Choke!! Knee on sternum!!! Armbar!!! To the point where I felt bad.
Now, before the righteous police come out, I realize that I am a dick for doing that. I was actually trying to help him in a weird way. But, I think my ego got the better of me and I ended up apologizing to him because I knew he would be hurting for a couple of days. But, at the same time by the end of the class he became so respectful and changed his attitude. So, it kinda worked. Point to all this is, you WILL run into people like me if you do those things; guys who have been training for a while and can't stand new guys who think they know shit or are "tough guys". On the flip side, I'm the first one to line up to help you out if you refrain from those things.
One last piece of advise is... STFU (shut the fuck up and train). Always remember that and you'll be around for a bit.