Revealing mistake: Peter Tuddenham was the talented voice behind most of the Blake's 7 computers - Zen, Orac & Slave in particular. In "Sarcophagus," Zen is under attack from the telepathic alien mind brought aboard, and its voice changes pitch, rate and timbre as it struggles to ward off the attack. During many of these moments Zen sounds exactly like Orac or Slave, since normally those computers' voices are Peter with the same type of pitch/rate tricks anyway.
Revealing mistake: To conceal their unauthorized entry into the secret lab, Cally very carefully closes and locks the sliding door. But when she turns back to speak with Tarrant, the "secured" door rolls halfway back open by itself. No, it's not Servalan's troops sneaking in to catch them: there's no sign of anyone there. Besides, anyone unlocking the supposedly-locked door would have alerted them. A case of malfunctioning BBC scenery, plain and simple.