That was Christopher Lloyd, and it was in ST III: The Search for Spock.
Oh yeah, I forgot about him. A three hour movie full of homely, indistinguishable mud-covered men bookended by two clean shaven handsome Hollywood stars.
Neither were cameos; both were significant parts.
How about Joe Piscopo in TNG: The Outrageous Okona…
John Tesh was one of the Holodeck Klingons who tested Worf on his birthday.
Yup. Even referred to as “K’Tesh” in the CCG!: Klingon Rite of Ascension Chamber holograms | Memory Alpha | Fandom
My wife still calls Giovanni Ribisi ‘Lightning Boy’ every time we see him on screen.
Johnny Carson showed up once as a train conductor taking tickets in a Get Smart episode.
Christian Slater was a big Star Trek fan as a kid, he messed up his eyebrows trying to look like Spock.
That looks a heck of a lot like Dave Thomasof Wendy’s fame…or not. 
As “Ray D. Tutto”, which was a joke - “Re di Tutto” (or “Tutti”, surely?) means “King of Everything”, a reference to a line in the film.
Christian Slater had a cameo in the first Austin Powers film too, complete with weird eyebrow action. And sherbet.
And speaking of Mike Myers, my favorite cameo ever: Charlton Heston in Wayne’s World 2. Possibly also my favorite Heston appearance ever.
David Bowie’s numerous strange cameos might need their own thread. He showed up as Pontius Pilate in The Last Temptation of Christ.
I just watched **The Holiday **for the second time last night. It’s a fun little romantic comedy in which Cameron Diaz and Kate Winslet swap houses for Xmas and fall in love with Jude Law and Jack Black, respectively. Nothing profound, but it’s a harmless way to pass the time. Anyhoo, Jack Black’s character is a composer for Hollywood, and he’s giving Kate Winslet a lesson in “Great Movie Scores 101” at a Blockbuster when he picks up The Graduate. At this point the camera cuts to Dustin Hoffman browsing in the same Blockbuster. It’s pretty clearly supposed to be him and not a character played by him, or at least it’s pretty clear to me. He simply glances sideways at Jack Black, smirks, and maybe mumbles a line or two under his breath, but the other two characters don’t even acknowledge his presence. It definitely fits the criteria for a strange cameo.
This wouldn’t really qualify as “strange,” but. . .
In the movie “Feds” (1988) Gordon Liddy shows up toward the end of the movie in an uncredited, non-speaking role. I remember him being a scene with Fred Thompson where Thompson is giving the FBI wannabes the facts of their first test mission.
Officer William Obanhein played himself in Alice’s Restaurant.
Not really unrealistic. The majority of the movie was set in the field during a battle. But the opening and closing scenes (where Travolta and Clooney appeared) were set behind the lines.
At the start of Into the Wild, the guy who gives the boots to the actor was the guy who in real life gave his boots to Chris McCandless.
Yeah. And as Andy Warhol, whom he knew pretty well, in Basquiat: http://www.wired.com/underwire/wp-content/gallery/david-bowie/bowie_basquiat.jpg
Jeff Goldblum in Annie Hall. He was on screen for like 5 seconds.
I saw Innerspace (1987) this weekend. There’s a scene in which grocery store cashier Martin Short is having trouble ringing up Kathleen Freeman’s *order.
The two customers behind her in line are Ron Howard’s dad Rance (who has shown up in bit roles forever)–but behind him is none other than legendary cartoon director extraordinaire Chuck Jones. He even has a line (referring to how much Short’s register is overcharging) “At these prices who would [buy that bottle of aspirins].”
*His register goes haywire and ends up ringing her buggy as over $100,000.