Diary of a Perfect Murder - S1-E1
Character mistake: Prosecutor "Death Row" Hawkins finishes speaking with prospective jury member Mr. Cooper and tags in The Defense attorney, saying, "Mr. Matlock, your witness." Naturally, Cooper is not a witness of anything; they are selecting the jury.
Other mistake: In a newspaper article about an overnight break-in and robbery at a bank, three paragraphs in the first column are repeated in the second column.
The Good Boy - S4-E3
Continuity mistake: When Cal Ritter's wife comes to his furniture studio to talk with him, the clock shows the time as 4:45. After they talk for two minutes, the security guard enters and says, "It's 5:00 and my ride's here. I just want to let you know I'm leaving."
Continuity mistake: Julia's hands go from being on her accomplice/boyfriend's cheeks to around his neck, then back to his cheeks in one shot.
Revealing mistake: Ben Matlock is hosting a pre-wedding party for his goddaughter. Several cars are parked on both sides of the street in front of Ben's house. Each time the front door is opened, The View seen from inside the house looking out is the same matte painting used in every episode. It shows a house and trees but no cars.
Factual error: This takes place in Atlanta, but in the scene where Tyler pulls up to Matlock and The Professor on the bench, there are California-style palm trees visible in the background. (00:18:35)
The Scandal - S9-E2
Character mistake: Billy has second thoughts about selling his late sister's beach house to a developer who plans to tear it down and build a hotel. Billy tells Ben, "She lived the last years of her life here." But in part 1 of this two-part episode, it was established that the woman had lived in the house for only six months.
The Singer - S7-E5
Character mistake: Near the end, Matlock says that he can prove who killed KC. The burden of proof lies with the prosecution; as a defense attorney, Matlock doesn't have to prove anything.
Answer: I did not see this episode, but YES. Both prosecution and defense lawyers use investigators to search for and uncover evidence that is legally admissible.