I haven't played much online, but I've read of at least one bug caused by modders. What should I look out for in multiplayer and how can I protect myself other than the usual either to not play online or to only play with people that I know would not do things that they know may negatively affect my game (though to define it so broadly, it's hard to imagine anyone to be such a saint)? For example, what are common bugs and trolls and what can I do if I am affected by them (other than the usual methods of abstinence)?
The main thing that i would look out for is anytime people ask to look at your item, there are a lot of players online that will try and get rare gear by simply stealing it from you.
Other things that i have seen include getting players into fights they can't win and then not helping so they die. presenting trades that are massively one sided against the player.
You will also run into hackers that you need to worry about, i find they ruin the game more than anything else - people out there who have invincibility shields and million damage rifles that just plow through the game while you do nothing just takes the fun out of it.
Bug wise the main one is item duping see this thread to see the main ways this is done Link this has also been used as a scam to take players gear in the promise it will get duped and then just stealing it.