When you were little, I bet your mom told you that. Mine did. It's good advice for Second Life, too. There are several "bad things" that people may try to give you:
- A URL to a "phishing" website that, on first glance, seems to be a Second Life web login page. Don't enter your user name and password information! If you do, you will have given them to the scammer, who can then log in as you, take your money, delete your account, or use it to commit Terms of Service violations that will get you banned.
- An account debiting object. When you rez it, it will request permission to take $L from you. If you are not paying attention and click "Allow", it can suck all the money out of your account. This may even be something that you have bought, rather than something someone gives you.
- A deformer. This could be something you wear, an object you sit on, or even a gesture. When used, it will deform your avatar into a bizarre shape, necessitating a re-log. If it's a gesture, it will re-deform you until you deactivate it.
- A griefer object. The "Bad Lab" HUD (so I'm told) will begin to automatically grief the region you are in when you put it on. It creates particle attacks and self-replicating physical megaprims (very laggy!) Since you are the one wearing the device, guess who will get AR'd for griefing? Yep...you!
- Stolen Content. People may pass stolen content to you in all innocence. This can be embarrassing when someone accuses YOU of being a copybotter. However, in most cases the worst that will happen is that LL will delete stolen content from your inventory and replace it with (fairly useless) generic items, if the creator has filed a DMCA takedown notice regarding the stolen items.
- A chat spy object. This can be almost anything, and while you're wearing it (or are within chat range, if it's something like furniture that you've put in your home), it will relay any local chat it "hears" to the person who's spying on you. You can get "listener detectors", but many objects have legitimate reasons to listen to local chat, so detecting an actual spy device can be tricky.
Now, I don't mean to say that you should NEVER accept things from strangers. People in Second Life are, generally speaking, generous souls and often hand out useful freebies to all and sundry. But do be aware that a few people DO give out these "poisoned candy" types of items, and exercise some caution!