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!"
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.