Continuity mistake: Ben joins Judge Carter Addison in his room in the first part of the episode. Addison is on the phone. Before he finally hangs up, he says, "Okay, I'll see you then." Before it cuts away, you can see Dick Van Dyke putting down the smoking pipe that is firmly between his teeth in the following close-up. (00:10:05)
Continuity mistake: Charlene comes back to the office after subpoenaing Piper Laurie. Ben is at his desk writing some notes. However, the pad turns 90° as the camera angles alternate. (00:19:50)
Continuity mistake: When Andy Griffith and Dick Van Dyke shake hands at the club, The Judge is holding his pipe at a different height in the separate shots. (00:24:00)
Continuity mistake: When The Judge is on the stand and Matlock approaches him to start his examination, Andy Griffith's right hand is on/off the railing, depending on the camera angle. (00:39:40)
Plot hole: The resolution of the case hinges on the fact that the culprit would be compelled by The Witness (whom they bribed, but who double-crossed them) to visit their stake-out apartment to try to dispose of their bloodied clothes. However, The Witness didn't and couldn't possibly know about that apartment. The murderer tried to prevent a non-existent threat that nobody made and did not worry at all about the other, very much more likely evidence and testimony The Witness provides - if The Witness was going to trick them to begin with, it makes no sense that they didn't just record the conversation, for once. The culprit does not even contemplate that possibility.
Answer: If it was ever found out that she was at Terry's house before he was killed, there's a possibility that a lot of people would have begun to suspect her of murdering Terry. By claiming that she wasn't, she was hoping that people wouldn't accuse her of the murder.