In "The Studio Job" Elliot (Christian Kane) becomes a minor country music star (playing "Thinking of You" a song from Kane's album The House Rules), basically a Captain Ersatz of his younger self called "Kenneth Crane" (The actor's initials reversed with a rhyming last name). There's even a fan site set up called "Craniak", fans of Christian Kane's music career are often called "Kaneiacs". Most Kaneiacs are female, and Elliot as Crane garners mostly female fans. Borders on Celebrity Paradox or perhaps No Celebrities Were Harmed.
Nate spends the majority of "The 10 Lil Grifters Job" trapped in an office while the rest of the team plays detective for him; just like his old boss Nero Wolfe. The episode was a somewhat missed opportunity as Nate's fictional detective wasn't Archie Goodwin, but it was made up for by the fact that Nate's fictional detective wasEllery Queen, one of the best-known TV roles of Timothy Hutton's father, Jim Hutton.
In that same episode, one of The Mark's employees calls him a vampire, and says he wants to put a literal stake through his heart. While talking to Eliot, i.e. Lindsey from Angel, who has some experience with vampires.
In "The Reunion Job", Hardison guesses that Eliot was the quarterback in high school. Ironic that it is Aldis Hodge who plays star high school QB Ray "Voodoo" Tatum in Friday Night Lights.
Saul Rubinek plays an executive at Bering Aerospace, likely accidental given that "The Nigerian Job" aired a year before Warehouse 13. However, in season four, he gets a minion named Latimer. Wordof God says it was unintentional, though.
Jonathan Frakes directed "The First Contact Job" (and plenty of other episodes). Eliot's alias in that episode is "Willie Riker," though Word of God says that this one was unintentional, since the Riker reference was written into the episode before Frakes was brought on to direct.
There's also "The Snow Job," which reunites Christian Kane with fellow Angel evil lawyer alum Sam Anderson, as well as Danny Strong from Buffy the Vampire Slayer. Not to mention Mark Sheppard popping up all over the place.
How can you look at Mark Sheppard without thinking of Crowley?
Timothy Hutton also played Archie Goodwin on A Nero Wolfe Mystery. Notable in that Kari Matchett- his ex-wife here- played an assortment of roles on Nero Wolfe as well, including Archie's girlfriend Lily Rowan.
British viewers might recognise Gina Bellman from Coupling, and a handful of people might notice that Aldis Hodge played Voodoo Tatum in Friday Night Lights. Or recognize him as Jake, the soldier who killed Sam from Supernatural.
Lampshaded in "The 12-Step Job" when Nate refers to an old gentleman in his rehab group as Nick Nolte. Uncredited, unconfirmed, but if it's not him, then it's a subversion rather than a lampshade, as none of the other cases are recognized.
The suicidal woman, the pretendung to be sick patient and a few other Scrubs alumni jioned the show.
Moreau, the Big Bad arms dealer the team is tracking in Season 3, turns out to be none other than Luka Kovac.
A few from "The Ten Li'l Grifters Job" - the mark is Mr. Matthews. His daughter is Jordan. The cop is Steven Flynn, who Firefly fans will recognize as the captain who attempts to steal Serenity in "Out of Gas."
Hey, look! Paul Kinsey is in "The 15 Minutes Job" and hasn't aged a day!
If one listens closely then he/she can figure out that the prison head from "The Jailhouse Job" is Spy, though his voice is the same as when he played the helicopter pilot. By extension he's also the Pyro.