A D.index error occurs when an invalid index is used when walking through an array or other data structure.
Many languages use zero-based arrays; that is, the valid indices for an array of size n go from 0 to n-1. This leads to a common indexing error when looping through such an array, starting at 1 instead of 0. (In the Python example here, note that range(1,n) includes numbers from 1 to n-1.)
for i in range(1, n): # code that processes array[i]
Beginning the indexing at 1 instead of 0 causes the code to miss the first element of the array. (In Python, you can write range(n) as a shortcut for range(0,n), which makes this error less likely to occur.) Similarly, you can make the same mistake on the other end, going past the end ...