in the way they handle non-comparable values:

const A = [1, undefined, -1];
d3.sort(A); // [-1, 1, undefined]
d3.sort(A, d => d); // [1, undefined, -1]

not sure if this should be considered a bug or not, but it can be surprising. The basic caveat is that having an undefined value (and after #210, a null), will throw a wrench into sorting—maybe it's something to add to the documentation instead.