- Complete Monster: In their reinvestigations UCOS have faced a variety of criminals, but these stand out:
- Ricky Hanson, the series's only recurring antagonist and Jack Halford's Arch-Enemy, is mid-tier London Gangster described by Jack as "the biggest thieving, murdering, lying piece of shyte I've ever laid eyes upon." Taking over the "Twenty-Fours" in the 80's, Hanson used them as a front for his drug ring. When Ray Harris told him that two student activists had infiltrated his group, Hanson personally murdered them, then had their bodies hidden under Harris's conservatory, incriminating him. By 1996, Jack and Sandra Pullman were investigating Hanson for three murders, including that of his own brother. Realizing they were getting too close, Hanson ran over Jack's wife Mary, laughing as he did; it took two years for Mary's wounds to kill her. When his son, Luke, was wrongly accused of arson, Ricky abandoned him to nine years in prison to save himself, and also stole Luke's girlfriend. In season 3's finale "Congratulations", Ricky confessed to Jack he killed Mary, tormenting him with the details of the event. In season 4's premier "Casualty", Ricky tried to slowly run Jack over but was forced to flee. Sneaking into Jack's hospital, Hanson assaulted the nurse then almost smothered Jack. Interrupted by Brian Lane, Hanson tried to strangle him. In season 6's "The Last Laugh", when the reinvestigation into the student's disappearance led back to him, Hanson set his goons on Gerry's informant.
- Alex Close, from season 7's "Dark Chocolate", was an immoral small-time businessman, whose companies operated through the exploitation and overwork of illegal immigrants. Close proved to be an even more vile man, as he was also a Serial Rapist who used his business's contracts to find victims. In 1992, he raped Jean Saunders at the chocolate factory where she worked, and then a few months later raped Eileen Harrison, permanently traumatising her. Close got around the mass DNA screening by forcing someone else to go in his place. When in 2001 his abuse of illegal immigrants was exposed, Close fled the country to Poland. Returning nine years later, he started a new business, this time using Polish immigrants, and immediately resumed his spree by attempting to get a bakery contract so he could rape his next target. When he failed, the undeterred Close resorted to regularly going as a customer, until he knew the layout well enough to rape Helen Vestry.
- George Mackie, from season 7's "Where There's Smoke", was a talented fire investigator respected for his dedication and knowledge. However, he was secretly also a serial arsonist, later claiming to having started sixty-four separate fires. He would place homemade firebombs in his targets, then after the fire burned out, he would turn up to analyze it and bask in the praise he received for his "insight". In 1996, one of Mackie's bombs caused the Union club to burn to the ground, killing four people, including the club's owner, and causing seven hospitalizations. Shortly afterwards, Mackie retired and thus the fires stopped. UCOS's reinvestigation of the Union fire inadvertently inspired Mackie to come out of retirement. Breaking into David Swallow's house, he set up another bomb; Swallow was himself a survivor of the Union Club fire who had been left with third-degree burns over half his body. Mackie then set another bomb in Stuart Russel's house, this one nearly killing Jack, Brian and Gerry along with Russel. Underneath all his charm and seeming helpfulness, at heart Mackie was nothing more than a shameless Attention Whore, happy to commit wanton destruction and kill innocent people just to stroke his own ego.
- Annabel Skinner, from season 10's "The Little Brother", was a former associate of the notorious Larry Goftmon. To steal Florence Epstein's money, she seduced Florence's husband Jonathan until he was so under her spell he agreed to murder Florence and steal her money. They tortured her bank details out of her, before bludgeoning Florence to death with a hammer. No longer needing him, Annabel turned Jonathan over to the police. However, Goftmon double-crossed her and ran off with the money. Desiring vengeance, not just against Goftmon but also the accountant Peter Sale—despite him being unaware of their plan and being cheated himself—she violently murdered Sale's sister Margaret Kirby and stole her identity. After six years of searching she found Goftmon, but couldn't find Sale. Devising a plan, she set up a trail directing towards him and got acquainted with Esther Lane. Playing the role of a caring sister who wanted to tell her brother he was an uncle, she thus sent Brian on the trail to find Sale. Skinner then tipped the police about Gofton's new alias, before strangling him and leaving the body for Brian to find. Finally finding Sale, Skinner disabled the police officer who was guarding him and tried to murder him. When Brian interrupted her, she tried to strangle him too. A manipulative violent sociopath, Skinner even described herself as "fundamentally nasty".
- Ear Worm: "It's all right, it's okay"
- Hilarious in Hindsight: In one early episode Gerry is told by a scrap metal merchant that his car, a classic Triumph Stag, is barely even worth scrap metal prices because no one buys them. The joke obviously being on Gerry, since he is only there on enquiries about the case, for driving a rubbish car. Since that episode was made, classic car prices have sky-rocketed and Triumph Stags have become retro-cool with even bad examples making serious money (this is partly due to the discovery that the "problems" Stag engines had was actually due to their radiators not being flushed properly by owners as part of the maintenance regime).
- It Will Never Catch On: TV critic AA Gill has been widely mocked for claiming the show would sink without trace when it first debuted; of course it has not only been going for more than a decade with very high viewing figures, but is also one of the most popular British export programmes.
- Magnificent Bastard: MI-5 agent Stephen Fisher (played by Tim McInnerny) in Series 9.
- Suspiciously Similar Substitute: Averted with Steve replacing Jack, as they are very different characters. Less averted for Dan replacing Brian, as they are both The Spock and have family issues (though different ones) and for Sasha replacing Sandra.
- They Changed It, Now It Sucks!: The reason Waterman left the show - when all the other members of the original cast had left the show Waterman went on record to say that he missed the chemistry he had with them and it's just not the same without them so he decided to move on too.
YMMV / New Tricks