The third person entering a conversation can change the meaning of the sentence already underway.

A sentence can change meaning halfway through because someone new walked in. The words were aimed at one listener and now land in a wider room, and what was candid becomes indiscreet or what was affectionate becomes performance. Meaning depends on the audience, and the audience can change mid-utterance.

Be aware of it in both directions. When someone joins, notice that you may need to restate or stop rather than pushing through. And when you walk into a conversation and something sounds odd, consider that you may have arrived in the middle of something that was not built for you to hear.