I mean mostly formally teaching Dharma. Presenting oneself as a Dharma teacher and all that jazz.
Nevertheless, even if "informally teaching", if there's such thing, one has to be as cautious. But others will not deposit the same amount of trust in us and will be less prone to have faith in what we say if we are not full fledged teachers, presented that way. So its safer for those listening. Mostly we are the ones in danger.
I guess in the end it boils down to knowing if we can really help others. Having a chat about Dharma if one knows the basics well enough, has a good grasp of theory and some well established practice seems fine. We need to be careful about our motivation and see how we react when people flatter us, criticize what we say and think or even speak badly of who we are. This goes for something as simple as chatting about Dharma (on and offline). I've seen people being really bad influences in informal banter about the teachings. This is pretty negative, IMO.
Having a conversation or helping a friend clarifying some doubts is not the same as being a a teacher. For instance, Namdrol doesn't present himself as a teacher (even though he has the qualifications to be one, IMO), but I've learned tons of stuff from his posts, sites and so on. I consider him a trustworthy source. Only if he says something that seems opposite to what I was taught I bother to check it. Otherwise I don't. The problem is that there are teachers with much, much less preparation than Namdrol that present themselves as such. This is a problem and their students will pay dearly most of the times...
I've seen one or two lamas losing their feet in matters that Namdrol dominates completely. We all have heard about scandals, sometimes related to people with tremendous amount of training. If these people can go wrong, imagine people with little training...
Do we know enough to clarify a doubt? Are our intentions wholesome? Can we deal with what happens during and after talking about Dharma with a group of friends or strangers? We need to have all this things sorted out before chatting about it if we want to play on the safe side.
So it's much safer to be a student until one is in fact ready to be a teacher. Students can help each other too, under the guidance of a good teacher.