It's all about trust with me. I trust very few people IRL. My personal little "rule" is once a backstabber always a backstabber. I will never trust someone again who has fucked me over or seriously hurt me in some form. Maybe I'll still talk to that person and spend time with them, but I will never actually TRUST them again. I learned this lesson very, very well during my first year of College, particularly towards the end where I learned who people are and what they will do.
Now everyone time I meet a new group of people I'm always thinking "what if this person tries to fuck me over" or "what's his story" or "me and this person aren't going to get along", and most of the time I'm right with my judgement.
1. I went to school with someone who we quickly became best friends when he was moved to my group. We talk for a few months and eventually he says his computer is broken and asks if I can fix it. I go round his house and fix it and everything is cool. As time goes on and I go round time after time I notice he becomes stale to me but suddenly super nice to me when his PC is broken. So basically he was only my friend when he wanted his PC fixed. I just dropped the whole "free repairs" thing for him and told him it would be £50 every time I go round or he can find someone else to do it. He didn't speak to me much after that.
2. I had one best friend I went to school with, year or so after we finish school I run into some stupid shit with some prick who I also went to school with. The guy gets pissed off and wants to know where I live, so he asks my "best friend" for my house address, and what does he do? He just tells him over the phone without even hesitating and that goes on to start a whole load of stupid shit with them coming to my house. I'm still best friends with him today, but I don't feel I can trust him again that much after just giving out my personal info like that.
3. I go to College with some person who's pretty fucking dumb. I buy a headset off him (I have no idea why I made the decision to buy it). I take it home and test it, it doesn't work, I ring him up and state it doesn't work and I want a refund. What do I hear? "oh no sorry didn't you hear me I said no refunds". I flip shit and want to beat the living daylights out of him the next day. I instead go in and just confront him in front of the class. He takes it and says he'll test it. Next week some other shit happens (explained below) and I just drop out and face the fact I won't be seeing that £30 again (I think some users here will recall my personal support thread in 2012 at the time. It's funny though, the guy who scammed me I recently found had been charged with drugs possession and has been studying some Level 1 courses in College for the past 2 years (basically a course level for stupid people). So what goes around comes around I suppose.
4. I also go to College with one guy who pretends to be my best friend in the whole class. Throughout the year he always talks about Tor and the other side of the web that terrorists use and all that stuff. Then he starts to 'turn' the conversations about me and what I do online. I'd have to write a whole page to explain the entire story. But long story short, he and his whole family (yes, his whole family) try to frame me for whatever extreme crimes you can think of in the United Kingdom. I remember it was stuff like illegal firearms possession, terrorism and apparent blackhat related stuff. After all that I've apparently been insulting the guy too and threatening to kill him too over Skype... After that some retarded shit is involved with them coming to the house and trying to kick the door in. They print off a copy of the chat logs and give them to my College tutors to look at, I go see the College later and they tell me they won't be contacting the police at that time but I should be careful for the future. Few days later police come round, they basically ask me for my side of the story on it all, I give it, they don't have a warrant so they can't take my computer. They leave and I never hear from them again. This is a much shortened down version of what happened, but I think you can tell basically what happened. I think this event in particular taught me a lot about trust and what information you share with people.
Remember anyone can backstab / scam whatever you want to call it at anytime in life or online. You just have to try and work them out and see who's trustworthy and who's just going to shoot you in the head when you turn your back (not literally of course but you get what I mean).
Don't ever disavow a best friend.