There is absolutely nothing wrong with walking into a room if you have an IC reason to be there... once you're there though, problems can potentially arise based on what you do from that point on. First off, there is the complaint that the newcomer is interrupting/disrupting/ distracting the ongoing roleplay. Then there's the complaint that the newcomer is acting in an unjustified manner. Then there's the complaint that the newcomer's presence is turning events in a unfavorable manner. These are just a few of the issues that rise into heated arguments when a newcomer stumbles into a scene, even when this happens with completely honest and unplanned intent.

So even if your intent is good, you could possibly end up being perceived in this fashion by the people involved in the pre-existing rp. So what can you do to avoid this?

It's actually really simple, and like many other RP-issues, avoiding being the lemming everyone is discussing on a channel can be handled fairly easily with just a bit of courtesy.