Corpsing: In 'The Modern Prometheus', where Amanda goes to Methos's place and they talk. Peter replaced his line with a joke during one take, and Elizabeth Gracen had a ton of trouble getting through the scene straightfaced after that.
Distracted by the Sexy: The director, during the Cassandra/Duncan love scene in "Prophecy". The blooper reel shows Tracy Scoggins finally saying loudly "If this goes on much longer, I'm going to need birth control!"
Troubled Production: A spin-off was conceived in Season Seven after Paul left to pursue movies. There was internal pressure to keep the franchise going without any delays, despite the writers and producers being unsure of where to go from there. After testing out a revolving door of potential Highlanderettes — Paul was not contractually obligated to appear for the whole season, and thus six or seven episodes were dedicated to his replacement(s) — they finally settled on Amanda (Elizabeth Gracen) to be "The Raven". Simple, right?
Unbeknown to Bill Panzer or his staff, Elizabeth Gracen had quietly gone off the deep end, believing she was being hunted down by the CIA and President Clinton (with whom she claimed to have had an affair). To top it off, Gracen had a new "manager" whom she met in France: a older fellow who claimed to be an Ambassador/Mob boss/business magnate, but was actually some sort of delusional stalker.
The music director kept missing his cues and went out drinking.
The film crew handled the series in a regimented, somewhat generic style which prolonged shooting.
To appeal to the European market, the French co-producers wanted Moonlighting-style light comedy. The showrunners wanted to stay true to the tone of Highlander.
Amanda wasn't believable as a tortured antihero, and discovered that her frisky antics had accidentally led to the deaths of over a hundred people.
The male co-star, Paul Johanssen (One Tree Hill) was gradually downgraded to Amanda's bumbling sidekick (aka Richie lite), to his dismay.
Amanda's roommate, originally written as a girl in her twenties, was changed to an old matron so as not to upstage Gracen; this resulted in Amanda's apartment being decorated like a retirement home.
The two leads detested each other, killing any romantic chemistry between the characters.
What Could Have Been: In the series, Hugh Fitzcairn was supposed to die at the end of the episode "The Hunters", but the crew ended up liking the character (and Daltrey) too much to just have him killed off.
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.
Peter Wingfield (Methos)began studying medicine before going into acting. Now, he's left acting to finish his schooling and become a medical doctor.
Clancy Brown was highly allergic to the latex makeup for The Kurgan's neck wound in the first film; it gave him problems during filming.
The series main cast got annoyed by the director getting annoyed at them eating bananas on the set and telling them to quit it. So, they showed up with 'no banana' tshirts once shortly after, with the typical red circle with a line through it over the fruit image. There's also a shot on the blooper reel of them eating bananas anyway, probably to tease him.
A couple of bloopers were left in because they seemed to flow well with the ep, like Duncan and Amanda's head butt while trying to kiss in 'Money No Object', and Duncan and Richie breaking out in laughter at the end of one early episode.