The Battlestar Galactica miniseries that kick-started Ronald D. Moore's re-imagining of the 1970s sci-fi show contains a contradictory plot hole involving Cylon biology. Moore's 21st century remake of Battlestar vastly updated the robotic Cylons of the original show, creating a more advanced, humanoid version. They looked like everyday humanoids, and each of the 12 models were supposed to be biologically indistinguishable from the human beings on the Twelve Colonies of Kobol.
The first humanoid Cylon model introduced in Moore's Battlestar Galactica was Number Six (Tricia Helfer) who was later revealed to be the lover of Gaius Baltar (James Callis). Number Six seduced Baltar and used him to infiltrate the Colonial Defense System ahead of the Cylons' devastating attack on humanity. Despite this, Gaius Baltar and Number Six would continue to be inextricably linked to each other throughout Battlestar. However, their first scene together contains a visual clue as to Six's true nature, which would create a Cylon plot hole further down the line.
Caprica Six's Spine Is A BSG Plot Hole
During Number Six's sex scene with Baltar, her spine is seen to glow with the same red light as seen on the mechanic Cylon Centurion's eyepieces. As the humanoid Cylons were designed to be biologically indistinguishable from humans, a glowing red spine is a glaring design flaw. It's also a plot hole, because surely the other 11 Cylon models could be easily detected in similar circumstances. However, at this stage in BSG, Moore hasn't conceived of the Final Five storyline that would become integral to the series.
It would later be revealed that the humanoid Cylons are organic creations that have superior strength and processing power, rather than mechanical augmentations. The red lights in Number Six's spine were swiftly forgotten about after the first episode of the miniseries. When the success of the miniseries led to a full Battlestar Galactica series, this glaring plot hole was glossed over. However, there was a very good reason for bending the rules about the humanoid Cylon's organic bodies.
Why Battlestar Had To Show Number Six As Different
Although Number Six is first introduced by emerging from the Cylon side of Armistice Station, flanked by two Centurions, it's not immediately clear that she's actually a Cylon. Therefore, during the sex scene with Baltar, Battlestar needs a quick visual cue to confirm what audiences already suspect. As the Cylons from the original Battlestar Galactica series were memorably designed with the blinking red light, it makes sense for Six's spine to contain something similar to indicate her true nature to the audience.
Quickly revealing that Baltar's lover is a Cylon sets up the massive security risk that the Caprican scientist has been party to. As Number Six tells the ambassador in the opening scene, it's already too late to avert disaster as "it has already begun." Seeing the red lights on Six's spine as she's in bed with Baltar is a literal red flag to warn the audience of the impending destruction that is about to befall the 12 Colonies. Although it's technically a plot hole, the glowing red spine was quickly forgotten about when Battlestar Galactica returned as a full series and doubled down on the humanoid Cylons organic origins.