Hey, It's That Guy!: WWE superstar Edge makes a brief cameo appearance in Highlander: Endgame. Almost counts as Retroactive Recognition as Edge has achieved far greater success since the film's release, but he was regularly featured on WWE programming when the film came out anyway. The cameo also leads to a Shout-Out to his WWE ringname, when after he's killed, he's told "It looks like you've lost your edge".
The show had a continuing parade of singers- Joan Jett, Roger Daltrey, Nia Peeples...
'Rowdy' Roddy Piper, of professional wrestling fame appeared in one episode.
The Jimmy Hart Version: JH version of Motley Crue's "Dr. Feelgood" plays during Connor's and Kane's final showdown in the third film.
Hey, It's That Place!: {{MerlinCamelot Castle}} is where the nobleman whose daughter Fitz was supposed to be protecting lived in 'Star Crossed'.
There are several Highlander scenes that were left-in goofs. At least two are at the end of eps, the one I reacall being "Prodigal Son". Adrian and Stan came out of character thinking the director should have already yelled 'cut'. There's also a scene later where Duncan and Amanda butt heads attempting to kiss, in "Easy Money" that was a goof left in.