Re: next-line is non-deterministic

[Richard Stallman]

    (progn (move-end-of-line 1) (next-line 1))

    produces different results depending on the method of moving the point
    to the beginning of the buffer before evaluating this expression.

This is because of the temporary goal column.
If the previous command was C-p, and it moved to
column zero, this call to next-line will also move
to column zero.

It is not a bug.