I am using Mathematica 10 now.

I typed code below

{{1, 2}, {2, 4}, {3, 6}, {4, 8}}[[;; , 1]][[1]]

and this gives result

1

But code

{{1, 2}, {2, 4}, {3, 6}, {4, 8}}[[;; , 1, 1]]

gives error

Part::partd: Part specification {{1,2},{2,4},{3,6},{4,8}}[[1;;All,1,1]] is longer than depth of object. >>

But according to help doc

expr[[i,j,...]]] or Part[expr,i,j,...]] is equivalent to expr[[i]][[j]]...

What is wrong?!! I remember I always coded it like this in Mathematica 9, no error occurs. Is it a bug??