I am sure this has been discussed before. It seems whenever I am in a nebula exploring 'unknown' systems, I will often come upon colonies whose inhabitants we already seem to know.
Yesterday I was exploring a nebula and my mission was to reset some alien devices that were causing hazardous emissions. The inhabitants also informed us that parties of Dref (I think that's who they were) were beaming down and we needed to deal with (i.e., kill) them.
Now, I can see Starfleet helping out a new species in distress by examining and resetting some alien devices. Fine. Good way to start a relationship. But no way am I going to start fighting another unknown species just because the first unknown species says they are bad.
The thing is, the information was presented to me by my bridge officer as if we already had knowledge of them. To me, 'unexplored' means we haven't been there yet. We don't know who or what is there. We shouldn't have any information at all. I realize this may seem kind of 'picky'.
Now, once in a while I can see going to an unexplored system only to find colonists from some species we do know. We just didn't know they were there. And, since we know about them, we may also know that they have an enemy who has tracked them there to do no good. But that should be the exception.