Mumbling
Witnesses sometimes do not really want to talk about things, especially things that cause them to be in a lawsuit. As most people do when they say something that they don't really want other people to hear, they mumble. When witnesses mumble and the court reporter cannot understand what they are saying, the stenographer will interrupt. They will tell the judge or the person's attorney that they cannot hear the witness or are unable to decipher what was said. Under most circumstances, they will normally not speak directly to the witness, because they are not involved in the proceedings. They are an observer that basically blends in to the background. When a clear record cannot be made, the words must be repeated so that it can be taken down, the court reporter can do their job of providing legal transcription services and the record will be complete.
Speed
Some people speak faster than a court reporter can take down what they are saying sound by sound and syllable by syllable. They may think it is funny after being after being asked to slow down. Some people even brag that they talk so fast that they make stenographers interrupt. Rather than helping to make a clear and accurate record, these individuals are instead putting their own words in jeopardy. One missed syllable can completely change the meaning of a word. For example, if a fast speaker said "it was unintentional," but spoke so quickly that everyone in the room heard "it was intentional," that could change the whole light of someone's testimony. Instead, the witness would be wise to slow down when asked, speak clearly and deliberately and help not only themselves, but the person reporting what they say and others in the room that are trying to understand every aspect of the legal case.
Overlapping
When court reporters take down multiple speakers in a proceeding, each one is identified in the record before their spoken words. Only one person can be taken at a time, as the court reporter follows the words of one speaker. They can stop that speaker and start another, but cannot take down two people at the same time. Sometimes people interrupt each other in normal conversation. This is not helpful in a deposition or court setting. If only one person's words are on the record, the other person talking is not included. This makes the record incomplete. Usually, the court reporter will interrupt the people talking overtop each other and ask them to speak one at a time. When people are arguing, they not only speak faster, but will run over each other's words as well. This make for extra difficulty in creating a reliable transcription of the proceedings.