This exchange, whilst Maddy is locked in a jail cell on suspicion of conspiracy:
Maddy: What if they give me some kind of truth serum?!
Jonathan: It wouldn't stand a chance.
Adam's attempt to cash in on the success of Derren Brown, which ends with him being attacked by an angry mob of persons of small stature. (Amazingly, It Makes Sense in Context.)
The episodes featuring Adam's attempts to play Follow the Leader with David Blaine are also strong candidates.
When Adam's latest Girl of the Week (an Austrian seamstress) misinterepts his need for a "frog-coat" for a Jack-the-Ripper sketch and ends up making him a giant frog costume, complete with paper-mache head and flippers for his feet. Even better is Jonathan's reaction to it:
Jonathan: Is this it then, is it? Through the swirling fog of Victorian London a young girl is slashed to ribbons by Toad of Toad Hall!
(Sounds like the most hilariously weird fanfiction premise ever, doesn't it?)
She also gives him a choice of shirts to wear for his act which are all black. Adam confides in Jonathan that he's after her because he's heard she's from an isolated 'very conservative' valley and therefore must be naive and easy to manipulate. The payoff comes at the end, when we find out just how conservative that valley is—she's a Nazi!
Maddy and Jonathan, mid-case, stopping to have an extended argument about the lyrics to the chorus of "My Friend the Witch Doctor". It almost qualifies as a Big Lipped Alligator Moment considering it has absolutely nothing to do with anything they've been discussing in the episode up until that point, and is never mentioned again. It also serves to illustrate the fact that despite their sniping at each other, the two of them are most happiest when they're squabbling over nothing.
The argument was probably born out the fact that one of the suspects in the case was heavily obsessed with voodoo, but yes - otherwise a totally random and pointless argument in the middle of nowhere.
Nigel Planer's cameo in "The Reconstituted Corpse" as Maddy's blind date. A stereotypical British gentleman, reminiscent of John Cleese, who briefly speaks of sex in a rather loquacious fashion. He also returns from the toilet in a restaurant with water all down the front of his trousers and loudly declares "It's not urine so it won't stain!"
Almost any time Adrian Edmondson is on-screen as Carla's pompous, pretentious but strangely loveable husband Brendan.
Maddy's flat is burgled and the lock on her door is taking a while to fix. In the meanwhile, she leaves a post-it note on the door to deter others: "Burglars, don't bother - I've been done." Jonathan remarks:
"Interesting security system."
And the burglar answered her phone for her when it went off while he was burgling her flat!
Jonathan: Oh no - he told me he was doing something with your computer.
Maddy: He was. He was stealing it!
The time when, due to not paying attention to what the man on the other end of the phone line was saying, Maddy ended up at a nudist meeting.
In "Black Canary", Rik Mayall cameos as DI Gideon Pryke who saves Jonathan from having his neck broken by a thug in a car park:
Thug: What's your game?
Pryke: (whipping his police badge out) This. What's yours, tiddlywinks?
In "The Curious Tale of Mr. Spearfish", Maddie has Jonathan do her laundry. This includes his ironing her bra.
In Angel Hair, Jonathon orders a ventriloquist's dummy for a new trick from a maker he notes is "slightly dotty". When he calls to confirm that the dummy will come with "12 feet" (spares since the trick involves sawing it in half), we cut to the maker, who is working on a 12 foot high dummy. The fact that the show actually made a freakishly large dummy for the gag makes it all the funnier.