Asymmetry

Asymmetry was the lack of symmetry between objects, parts or aspects.

The First Doctor noted that the Glass Avatar on board the Testimony ship had a face which was asymmetrical. The Twelfth Doctor, despite using his sonic sunglasses, failed to notice this, leading his younger self to criticise his reliance on technology. (TV: Twice Upon a Time)

When travelling with John and Gillian, the Second Doctor encountered a group of Asymmetrical Daleks. (COMIC: Attack of the Daleks)

Heather and Bill Potts encountered a sentient oil puddle, both noted the unusualness of their reflections. While Heather quickly figured out that the puddle was showing their non-mirrored faces, Bill did not realise this until she showed the puddle to the Twelfth Doctor, the Time Lord deducing that Heather had figured it out due to her asymmetric eyes. (TV: The Pilot)