It seems like you can never be fully prepared for distractions while speaking or giving a presentation. Over the last few years, I’ve experienced everything from loud coughing to a person walking across the room to ask someone else a question WHILE I was speaking.
Not calling attention to such things takes experience and practice – it’s the mark of a true professional. Most audiences are courteous and respect the speaker, so it may take years of speaking before you run into major distractions. But the first time it happens, you find it breaks your concentration to the point where it negatively affects the rest of your speech.