The Haunted Heist is a short film which was first featured in the American version of Daffy Duck and Porky Pig Meet the Groovie Goolies. However, its inclusion seems random and it's likely that it was shot for an unused segment in Sabrina and the Groovie Goolies.
In an article published in a 1970 edition of "Cashbox" just prior to the show's premiere, they discussed the live-action music sequences in The Hardy Boys and noted that "the Goolies will be live all the way." However, the live-action characters were nowhere to be seen in the original run of the show, and the copyright date on "The Haunted Heist appears to be 1975, which is when the Groovie Goolies were in reruns on ABC.
In the stand-alone version of the short, the Goolies are preparing for a rehearsal, but Wolfie can't find his guitar anywhere. Suddenly, Frankie notices Hauntleroy walking around with the guitar slung over his shoulder. Drac demands the youngster to return it, but Hauntleroy instead hops through the mirror and becomes a live-action character portrayed by musician Emory Gordy Jr.
"Man, the little brat's run away to Mad Mirror Land!" exclaims Wolfie.
The monster trio follow Hauntleroy through the mirror and are magically transformed into Dick Monda (Drac), Ed Fournier (Frankie), and Jeffrey Thomas (Wolfie), the same guys who appeared on the jacket of the 1970 LP (and at least two of whom provided the show's music).
Hauntleroy jumps into a tiny car, and the Goolies board a set of invisible wheels to follow via a stop-motion sequence. Hauntleroy throws out a piece of gum, which sticks to Drac and causes a three-Goolie-pileup.
Hauntleroy runs into a barn and jumps on a horse, with the monstrous adults following behind on invisible steeds. The Goolies are thrown into a puddle by their unseen horses, and then Frankie smashes his fist against the ground, which causes the earth to shake and Hauntleroy to accidentally throw the guitar back at Wolfie.
The trio then (sort of) perform Little Texas Goolie during a montage featuring too many visual stop-motion shenanigans to try to describe.
Eventually, (and for no discernible reason) the trio wind up sneezing, which pushes everyone back through the mirror. Once inside Horrible Hall, Hauntleroy slides all the way out of a window and lands in the moat, where he faces the wrath of a dragon.
The version in the Daffy Duck movie runs a little shorter and is edited slightly differently. There's a cutaway to Foghorn Leghorn, Sylvester and Pepe LePew watching the action unfold from the animated side of the mirror, as well as a variety of random cutaways to a pasture full of cows.
Although the Goolies appear to be playing music at one point, no music is audible.
The clip ends with the Goolies going back through the mirror and doing a Scooby-Doo unmasking Hauntleroy, who is actually The Phantom of the Flickers.
This entire live-action sequence was omitted from the European video releases of the film, which reinforces the notion that it may have been a preexisting scene that was tacked on.