One of the most disruptive behavior is when newcomers enter in the middle of a conversation, and immediately begin to offer opinions and even question the purpose of the conversation. This happens more than we’d like in the corporate environment. It is perpetuated by a culture that accommodates disruptive behavior. Co-workers stop to explain to the newcomer. No one stops to ask who invited the newcomer to the conversation, and whether the newcomer took the time to get acquainted with the conversation before barging in.
The practical way to keep emotions under check is to ask whether the newcomer is critical to decision making. If yes, it was a mistake to omit him or her in the first place. The meeting needs to be stopped until the newcomer is brought up to speed. If however, the newcomer is just someone who speaks loudly and sounds important, he or she need to be politely escorted out of the meeting or asked to observe silently until he or she is up to speed.
If the newcomer is a new hire, the disruption is clearly bad behavior from a person who thinks he or she know it all. Even if he or she knows it all, the proper thing to do would be to understand how best to intervene, and then speak up. Very rarely is a newcomer brought into a life and death conversation and not prepped in advance. That is bad behavior of a different sort.