Doctor Who Series 10: 11 Easter Eggs & References You Might've Missed In 'Thin Ice'
7. Stepping On Butterflies
Thin Ice marks Bill's first trip into the past, which causes her to worry about changing the future. "If I step on a butterfly, it could send ripples through time that could mean that I wasn't born in the first place."
She is, of course, referencing the Butterfly Effect, a concept that was first attributed to time travel theory in Ray Bradbury's short story A Sound of Thunder (which later became a film starring Edward Burns).
The last companion of the Doctor's to refer to it was Martha Jones. In her first trip to the past, in 2007's The Shakespeare Code, she worried if stepping on a butterfly would "change the future of the human race."
Martha also questioned if she would be OK walking around Elizabethan England, seeing as she "wasn't exactly white." Likewise, Bill worries the same thing here, and unfortunately does encounter some racism along the way.