Beast Boy and Robin cut the pizza in half this time that triggers time waves that change the present bit by bit similar to how in A Sound of Thunder when accidental killing of a butterfly literally triggers time waves that change the present bit by bit.
At around 4m 40s, references the character Brubaker -"...today, you stood shoulder to shoulder with Columbus discovering America, Armstrong stepping on the Moon, Brubaker landing on Mars..."
Martha asks what will happen if she stamps on a butterfly, and changes the future, making the Doctor ask what she's got against butterflies. She seems to have gotten the idea from the film adaptation rather than the original story.